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{{AssessPsy}}
 
{{AssessPsy}}
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{{Human intelligence}}
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An '''intelligence quotient''' or '''IQ''' is a score derived from one of several different [[intelligence measures]], [[standardized test]]s designed to measure [[intelligence]]. The term "IQ," a translation of the [[German language|German]] ''Intelligenz-Quotient'', was coined by the German psychologist [[William Stern]] in 1912 as a proposed method of scoring early modern children's [[intelligence testing|intelligence tests]] such as those developed by [[Alfred Binet]] and [[Theodore Simon]] in the early 20th Century.<ref>''i.e.'' as a [[quotient]] of "mental age" and "chronological age."</ref> Although the term "IQ" is still in common use, the scoring of modern IQ tests such as the [[Wechsler Adult Intelligence Scale]] is now based on a projection of the subject's [[Bell curve grading|measured rank]] on the [[Normal distribution|Gaussian bell]] curve with a center value (average IQ) of 100, and a [[standard deviation]] of 15 (different tests have various standard deviations, the [[Stanford-Binet IQ test]] has a standard deviation of 16).
   
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IQ scores have been shown to correlate with such factors as [[morbidity]] and [[mortality]],<ref>{{Cite web
[[Image:6sigmaIQrange.png|thumb|350px|IQ tests are designed to give approximately [[normal distribution|normally distributed]] results, which causes a [[normal distribution|"bell curve"]] graph of IQ score frequency. Colors delineate one [[standard deviation]].]]
 
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|url=http://www.jnnp.com/cgi/content/abstract/75/8/1100
An '''intelligence quotient''' or '''IQ''' is a score derived from a set of [[standardized test]]s developed to measure a person's [[cognition|cognitive abilities]] ("[[intelligence (trait)|intelligence]]") in relation to their age group. An IQ test does not measure intelligence the way a ruler measures height (absolutely), but rather the way a [[racing|race]] measures speed (relatively).
 
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|title= Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype
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|accessmonthday=August 6 |accessyear=2006
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|date=2004
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|author=Cervilla et al
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|publisher=Psychiatry 2004;75:1100-1106.
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}}</ref> parental social status,<ref>[http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf Intelligence: Knowns and Unknowns] (Report of a Task Force established by the Board of Scientific Affairs of the American Psychological
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Association - Released August 7, 1995 -
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A slightly edited version was published in the American Psychologist, Feb 1996. Official Journal of the APA)</ref> and to a substantial degree, parental IQ: while IQ heritability has been investigated for nearly a century, controversy remains as to how much is heritable, and the mechanisms for heritability are still a matter of some debate.<ref>[http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=9242404&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus The heritability of IQ. ] Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):468-71.</ref><ref>The same study suggests that the [[heritable]] component of IQ becomes more significant with age.</ref>
   
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IQ scores are used in many contexts: as predictors of [[education]]al achievement or special needs, by social scientists who study the distribution of IQ scores in populations and the relationships between IQ score and other variables, and as predictors of job performance and [[income]].<ref>{{Cite web
For people living in the prevailing conditions of the developed world, IQ is highly [[heritability|heritable]], and by adulthood the influence of family environment on IQ is undetectable. IQ test scores are correlated with measures of brain structure and function, as well as performance on simple tasks that anyone can complete within a few seconds.
 
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|url=http://www.neurology.org/cgi/content/abstract/55/10/1455
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|title= Childhood mental ability and dementia
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|accessmonthday=August 6 |accessyear=2006
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|date=2000
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|author=Whalley ''et al.''
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|publisher=Neurology 2000;55:1455-1459.
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}}</ref><ref>{{Cite web
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|url=http://archpsyc.ama-assn.org/cgi/content/abstract/63/11/1238
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|title= Intelligence and Other Predisposing Factors in Exposure to Trauma and Posttraumatic Stress Disorder. A Follow-up Study at Age 17 Years
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|accessmonthday=August 6 |accessyear=2006
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|date=11, November 2006
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|author= Naomi Breslau, PhD; Victoria C. Lucia, PhD; German F. Alvarado, MD, MPH
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|publisher= Arch Gen Psychiatry. 2006;63:1238-1245
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}}</ref><ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1572949&dopt=Citation Effects of major depression on estimates of intelligence] Sackeim HA, Freeman J, McElhiney M, Coleman E, Prudic J, Devanand DP. J Clin Exp Neuropsychol. 1992 Mar;14(2):268-88.</ref><ref>[http://www.blackwell-synergy.com/doi/abs/10.1111/j.1440-1819.2006.01564.x Improvement of cognitive functioning in mood disorder patients with depressive symptomatic recovery during treatment: An exploratory analysis] LAURA MANDELLI, Psy. D, ALESSANDRO SERRETTI, MD, CRISTINA COLOMBO, MD, MARCELLO FLORITA, Psy. D, ALESSIA SANTORO, Psy. D, DAVID ROSSINI, MD, RAFFAELLA ZANARDI, MD and ENRICO SMERALDI, MD. Psychiatry and Clinical Neurosciences Volume 60 Issue 5 Page 598 - October 2006</ref><ref>{{Cite web
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|url=http://www.bmj.com/cgi/content/abstract/322/7290/819
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|title=Longitudinal cohort study of childhood IQ and survival up to age 76
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|accessmonthday=August 6 |accessyear=2006
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|date=2001
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|author=Whalley and Deary
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|publisher=British Medical Journal 2001, 322:819-819
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}}</ref><ref>{{Cite web
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|url=http://www.ajph.org/cgi/content/abstract/AJPH.2005.080168v1
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|title=Associations Between Childhood Intelligence and Hospital Admissions for Unintentional Injuries in Adulthood: The Aberdeen Children of the 1950s Cohort Study
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|accessmonthday=January 10 |accessyear=2007
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|date=2006
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|author= Debbie A. Lawlor, University of Bristol, Heather Clark, University of Aberdeen, David A. Leon, London School of Hygiene & Tropical Medicine
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|publisher=American Journal of Public Health, December 2006}}</ref>
   
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The average IQ scores for many populations were rising at an average rate of three points per decade during the 20th century with most of the increase in the lower half of the IQ range: a phenomenon called the [[Flynn effect]]. It is disputed whether these changes in scores reflect real changes in intellectual abilities, or merely methodological problems with past testing.
IQ is correlated with academic success; it can also predict important life outcomes such as job performance, socioeconomic advancement, and "social pathologies". Recent work has demonstrated links between IQ and health, longevity, and functional literacy.
 
 
 
==History==
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== History ==
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In 1905 the French psychologist [[Alfred Binet]] published the first modern intelligence test, the Binet-Simon intelligence scale. His principal goal was to identify students who needed special help in coping with the school curriculum. Along with his collaborator [[Theodore Simon]], Binet published revisions of his intelligence scale in 1908 and 1911, the last appearing just before his untimely death.
   
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In 1912, the German psychologist [[William Stern]] coined the abbreviation "I.Q.", a translation of the German ''Intelligenz-Quotient'' ("intelligence [[quotient]]"), proposing that an individual's intelligence level be measured as a [[quotient]] of their estimated "[[mental age]]" and their chronological age. A further refinement of the Binet-Simon scale was published in 1916 by [[Lewis M. Terman]], from [[Stanford University]], who incorporated Stern's proposal, and this [[Stanford-Binet Intelligence Scale]] formed the basis for one of the modern intelligence tests that remains in common use.
Early attempts of mental tests were those of Sir Galton (1863) and James Cattell (1888). These tests were more physical tests than mental ones. Their importance was in developing the idea that one's IQ can be measured and is different from person to person. They also proposed normal distributions of mental tests results within a large population.
 
   
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Originally, IQ was calculated as a ratio with the formula <math>100 \times \frac{\text{mental age}}{\text{chronological age}}.</math>
[[Alfred Binet]] and his colleague [[Theodore Simon]] created the '''Binet-Simon scale''' in [[1905]], which used testing to identify students who could benefit from extra help in school. Their assumption was that lower scores indicated the need for more teaching, not an inability to learn. This interpretation is still held by some modern experts.
 
   
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A 10-year-old who scored as high as the average 13-year-old, for example, would have an IQ of 130 (100*13/10).
Notably, Binet himself made no claim that his test properly measured intelligence. He stated in his paper ''New Methods for the Diagnosis of the Intellectual Level of Subnormals'' that
 
:"This scale properly speaking does not permit the measure of the intelligence, because intellectual qualities are not superposable, and therefore cannot be measured as linear surfaces are measured, but are on the contrary, a classification, a hierarchy among diverse intelligences; and for the necessities of practice this classification is equivalent to a measure." [http://psychclassics.yorku.ca/Binet/binet1.htm]
 
   
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In 1939 [[David Wechsler]] published the first intelligence test explicitly designed for an adult population, the [[Wechsler Adult Intelligence Scale]], or WAIS. Subsequent to the publication of the WAIS, Wechsler extended his scale for younger ages, creating the [[Wechsler Intelligence Scale for Children]], or WISC. The Wechsler scales contained separate subscores for verbal and performance IQ, thus being less dependent on overall verbal ability than early versions of the Stanford-Binet scale, and was the first intelligence scale to base scores on a standardized [[normal distribution]] rather than an age-based quotient: since age-based quotients worked only for children, this method was replaced by a projection of the measured rank on the [[Normal distribution|Gaussian bell]] curve using an average IQ of 100 as the center value and a [[standard deviation]] of 15 or occasionally 16 or 24 points.
In [[1910]], [[Henry H. Goddard]] proposed three categories for the "feeble-minded" based on IQ scores: [[Moron (psychology)|moron]] (IQ of 51&ndash;70), [[imbecile]] (IQ of 26&ndash;50), and [[idiot]] (IQ of 0&ndash;25). This taxonomy was the standard of intelligence research for decades.
 
   
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Thus, the modern IQ score is a mathematical transformation of a raw score on an IQ test, based on the rank of that score in a normalization sample,<ref>see: [[quantile]], [[percentile]], [[percentile rank]].</ref> Modern scores are sometimes referred to as "deviance IQ", while older method age-specific scores are referred to as "ratio IQ".
In [[1916]], [[Stanford University]] psychologist [[Lewis Terman]] released the "Stanford Revision of the Binet-Simon Scale", generally known as the '''[[Stanford-Binet test]]'''. This became the most commonly administered test for many decades. The term "intelligence quotient," in which each student's score was the quotient of his or her tested mental age with his or her actual age, was adopted by Terman from a [[1912]] proposal by German psychologist [[William Stern]]. This led to refined testing developed by [[Robert Yerkes]] for [[United States Army]] recruits.
 
   
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The two methodologies yield similar results near the middle of the bell curve, but the older ratio IQs yielded far higher scores for the intellectually gifted&mdash; for example, [[Marilyn vos Savant]], who appeared in the Guinness Book of World Records, obtained a ratio IQ of 228. While this score could make sense using Binet's formula (and even then, only for a child), on the Gaussian curve model it would be an exceptional 7.9 standard deviations above the mean and hence virtually impossible in a population with a normal IQ distribution (see [[normal distribution]]). In addition, IQ tests like the Wechsler were not intended to reliably discriminate much beyond IQ 130, as they simply do not contain enough exceptionally difficult items.<sup>[http://www.minddisorders.com/Py-Z/Wechsler-Intelligence-Scale-for-Children.html]</sup>
Today, the most commonly administered IQ test is the '''[[Wechsler Intelligence Scale for Children|WISC-III]]''' test, originally developed by [[David Wechsler]] in 1974. The WISC-III test comprises ten types of problems, categorized by difficulty and by skill type (verbal and performance scales). A revised version, the WISC-IV, was released in 2003 and is used regularly in assessments. However, the interpretation of various combinations of subscales is still being researched. Another notable type of IQ test is the Bailey Scale of Infant Development, regarded as the 'best' means of testing cognitive development in infants.
 
   
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Since the publication of the WAIS, almost all intelligence scales have adopted the normal distribution method of scoring. The use of the normal distribution scoring method makes the term "intelligence quotient" an inaccurate description of the intelligence measurement, but "I.Q." still enjoys colloquial usage, and is used to describe all of the intelligence scales currently in use. The third edition of the WAIS (WAIS-III) is the most widely-used psychological test in the world, and the fourth edition of the WISC (WISC-IV) is the most widely used intelligence test for children.{{Fact|date=December 2007}}
Today, informal online IQ tests are popular, but they are at best rough approximations. The tests are not expert certified and notable limitations include a small number of questions and a lack of the time limit.
 
   
==IQ score distribution==
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==IQ testing==
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===Structure===
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;" ALIGN="right"
 
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IQ tests come in many forms, and some tests use a single type of item or question, while others use several different subtests. Most tests yield both an overall score and individual subtest scores.
|+ '''IQ score distribution'''
 
|- bgcolor=#ccccff
 
!IQ Range !! Percentile Range
 
|- bgcolor=E9E8FF
 
|Below 30 || 0 &ndash; 0.0002
 
|- bgcolor=#DFE0FF
 
|30 to 50 || 0.0002 &ndash; 0.04
 
|- bgcolor=E9E8FF
 
|50 to 60 || 0.04 &ndash; 0.4
 
|- bgcolor=#DFE0FF
 
|60 to 74 || 0.4 &ndash; 4
 
|- bgcolor=E9E8FF
 
|74 to 89 || 4 &ndash; 23
 
|- bgcolor=#DFE0FF
 
|89 to 100 || 23 &ndash; 50
 
|- bgcolor=E9E8FF
 
|100 to 111 || 50 &ndash; 77
 
|- bgcolor=#DFE0FF
 
|111 to 120 || 77 &ndash; 91
 
|- bgcolor=E9E8FF
 
|120 to 125 || 91 &ndash; 95
 
|- bgcolor=#DFE0FF
 
|125 to 132 || 95 &ndash; 98
 
|- bgcolor=E9E8FF
 
|132 to 137 || 98 &ndash; 99.3
 
|- bgcolor=#DFE0FF
 
|137 to 150 || 99.3 &ndash; 99.96
 
|- bgcolor=E9E8FF
 
|above 150 || 99.96 &ndash; 100
 
|}
 
   
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A typical IQ test requires the test subject to solve a fair number of problems in a set time under supervision. Most IQ tests include items from various domains, such as short-term memory, verbal knowledge, spatial visualization, and perceptual speed. Some tests have a total time limit, others have a time limit for each group of problems, and there are a few untimed, unsupervised tests, typically geared to measuring high intelligence. The most widely used standardized test for determining IQ is the [[Wechsler Adult Intelligence Scale|WAIS (Wechsler Adult Intelligence Scale-Third Edition]]){{Fact|date=November 2007}}. The WAIS-III consists of fourteen subtests, seven verbal (Information, Comprehension, Arithmetic, Similarities, Vocabulary, Digit Span, and Letter-Number Sequencing) and seven performance (Digit Symbol-Coding, Picture Completion, Block Design, Matrix Reasoning, Picture Arrangement, Symbol Search, and Object Assembly).
IQ scores are expressed as a number normalized so that the average IQ in an age group is 100. In other words, an individual scoring 115 is above average when compared to people in the same age group. It is common practice to standardize so that the [[standard deviation]] (&sigma;) of scores is 15, although some IQ tests use difference scales (for example, the Stanford Binet IQ test uses a standard deviation of 16, and the [[Cattell IIIB]] test uses a standard deviation of 24). Tests are designed so that the distribution of IQ scores is [[normal distribution|Gaussian]]; that is, it follows a [[bell curve]]. A difference has been documented between the IQ score distributions of left-handed and right-handed test subjects; the distribution in left-handed people tends to cluster at the two extremes of the IQ scale.<!--ref-->
 
   
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===Scoring===
(The following numbers apply to IQ scales standard deviation σ = 15.) Roughly 68% of the population has an IQ between 85 and 115. The "normal" range, or range between &minus;2 and +2 standard deviations from the mean, is between 70 and 130, and contains about 95% of the population. An accurate score below 70 may indicate [[mental retardation]], and a score above 130 may indicate [[gifted|intellectual giftedness]]. Retardation may result from normal variation or from a genetic or developmental malady; analogously, some otherwise normal people are very short, and others have [[dwarfism]]. Giftedness appears to be normal variation; [[autistic savant|autistic savants]] have often astonishing cognitive powers but below-average IQ's.
 
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When standardizing an IQ test, a representative sample of the population is tested using each test question. IQ tests are calibrated in such a way as to yield a normal distribution, or "bell curve". Each IQ test, however, is designed and valid only for a certain IQ range. Because so few people score in the extreme ranges, IQ tests usually cannot accurately measure very low and very high IQs.
 
It has been observed that scores outside the range 55 to 145 must be cautiously interpreted because there are smaller numbers of respondents with which to make comparisons in those ranges. Moreover, at such extreme values, the normal distribution is a less accurate estimate of the true IQ distribution.
 
   
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Various IQ tests measure a standard deviation with a different number of points. Thus, when an IQ score is stated, the standard deviation used should also be stated.
In actuality there is a higher percentage of the population measured at 3 or more standard deviation levels on the test than the probabilities of the normal distribution would predict. Some IQ scoring procedures may attempt to integrate such clusters of statistical outliers into the curve by adjusting the scores so that they better represent actual probabilities (according to Silverman) and in these cases, scores around 145 and above may actually have been notably higher, were they not so adjusted.
 
   
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When an individual has scores that do not correlate with each other, there is a good reason to suspect a learning disability or other cause for this lack of correlation. Tests have been chosen for inclusion because they display the ability to use this method to predict later difficulties in learning.
Most IQ tests in the [[United States]] tend to use a SD-15 or SD-16 scale, meaning that one standard deviation corresponds to +/- 16 points on the IQ scale. However, European IQ tests tend to use a SD-24 or SD-25 scale, resulting in discrepancies. Therefore, an IQ of 130 (+2 standard deviations) in the U.S. might correspond to an IQ of 148-150 in Europe. Due to these differences, percentiles are more accurate measurements than IQ numbers.
 
   
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An individual's IQ score may or may not be stable over the course of the individual's lifetime.<ref name="Neisser95">{{Cite web
==IQ and General Intelligence Factor''==
 
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|url=http://www.lrainc.com/swtaboo/taboos/apa_01.html
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|title=Intelligence: Knowns and Unknowns
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|accessmonthday=August 6 |accessyear=2006
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|date=[[August 7]], [[1995]]
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|author=Neisser ''et al.''
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|publisher=Board of Scientific Affairs of the [[American Psychological Association]]
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}}</ref>
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=== IQ and general intelligence factor ===
 
{{main|General intelligence factor}}
 
{{main|General intelligence factor}}
   
Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking, etc.), with the summary score calculated from subtest scores. Individual subtest scores tend to [[correlation|correlate]] with one another, even when seemingly disparate in content. Analyses of an individual's scores on the subtests of a single IQ test or the scores from a variety of different IQ tests (e.g., [[Stanford-Binet]], [[WISC-R]], [[Raven's Progressive Matrices]] and others) will reveal that they all measure a single common factor and various factors that are specific to each test. This kind of [[factor analysis]] has led to the theory that underlying these disparate cognitive tasks is a single factor, termed the [[general intelligence factor]] (or ''g''), that corresponds with the common-sense concept of intelligence. In the normal population, ''g'' and IQ are roughly 90% correlated and are often used interchangeably.
+
Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking), with the summary score calculated from subtest scores. The average score, according to the bell curve, is 100. Individual subtest scores tend to [[correlation|correlate]] with one another, even when seemingly disparate in content.
   
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Mathematical analysis of individuals' scores on the subtests of a single IQ test or the scores from a variety of different IQ tests (e.g., [[Stanford-Binet]], [[WISC-R]], [[Raven's Progressive Matrices]], [[Cattell Culture Fair III]], Universal Nonverbal Intelligence Test, Primary Test of Nonverbal Intelligence, and others) find that they can be described mathematically as measuring a single common factor and various factors that are specific to each test. This kind of [[factor analysis]] has led to the theory that underlying these disparate cognitive tasks is a single factor, termed the [[general intelligence factor]] (or ''g''), that corresponds with the common-sense concept of intelligence.<ref>{{Cite web|url=http://www.psych.utoronto.ca/~reingold/courses/intelligence/cache/1198gottfred.html
==Genetics vs environment==
 
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|title=The General Intelligence Factor
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|accessmonthday=August 6 |accessyear=2006
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|date=[[1998-11]]
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|author=Linda S. Gottfredson
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|publisher=Scientific American
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}}</ref> In the normal population, ''g'' and IQ are roughly 90% correlated and are often used interchangeably.
   
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Tests differ in their ''g''-loading, which is the degree to which the test score reflects ''g'' rather than a specific skill or "group factor" (such as verbal ability, spatial visualization, or mathematical reasoning). ''g''-loading and validity have been observed to be related in the sense that most IQ tests derive their validity mostly or entirely from the degree to which they measure ''g''.{{Fact|date=July 2007}} <!-- <ref>(Jensen 1998)</ref> Not a reference, please provide link to the source. -->
The role of genes and environment (nature vs. nurture) in determining IQ is reviewed in Plomin ''et al.'' (2001, 2003). The degree to which genetic variation contributes to observed variation in a trait is measured by a statistic called [[heritability]]. Heritability scores range from 0 to 1, and can be interpreted as the percentage of variation (e.g. in IQ) that is due to variation in genes. [[Twin study|Twins studies]] and adoption studies are commonly used to determine the heritability of a trait. Until recently heritability was mostly studied in children. These studies yield an estimate of heritability of 0.5; that is, half of the variation in IQ among the children studied was due to variation in their genes. The remaining half was thus due to environmental variation and measurement error. A heritability of 0.5 implies that IQ is "substantially" heritable. Studies with adults show that they have a higher heritability of IQ than children do and that heritability could be as high as 0.8, though it is probably not this high. The [[American Psychological Association]]'s 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the White population the heritability of IQ is “around .75” (p. 85).[http://www.lrainc.com/swtaboo/taboos/apa_01.html]
 
   
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===Mental handicaps===
Considerable research has focused on biological correlates of ''g''; see [[General intelligence factor]] and the section on brain size below. For example, general intelligence and [[MRI]] brain volume measurements are correlated, and the effect is primarily determined by genetic factors.
 
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{{main|Mental retardation}}
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Individuals with an unusually low IQ score, varying from about 70 ("Educable Mentally Retarded") to as low as 20 (usually caused by a neurological condition), are considered to have developmental difficulties. However, there is no true IQ-based classification for [[Developmental disability|developmental disabilities]]. But newer studies have proved that even though the perception of mental retardation in current social settings is of or tests as lower intelligence, some patients do show advanced abilities in terms of emotions, spatial abilities,cognitive and memory. Aside from current Savants and patients with Aspergers many children with Down Syndrome show a more introverted state of emotion but yet show uncanny ability between ethical and moral dilemmas.
   
===Environment===
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== Heritability ==
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{{main|Inheritance of intelligence}}
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The role of genes and environment (nature and nurture) in determining IQ is reviewed in Plomin ''et al.'' (2001, 2003).<ref name="Plomin0103">Plomin ''et al.'' (2001, 2003)</ref>. Until recently heritability was mostly studied in children. Various studies find the heritability of IQ between 0.4 and 0.8 in the [[United States]];<ref>{{cite journal
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| author = R. Plomin, N. L. Pedersen, P. Lichtenstein and G. E. McClearn
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| year = 1994
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| month = 05
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| title = Variability and stability in cognitive abilities are largely genetic later in life
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| journal = Behavior Genetics
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| volume = 24
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| issue = 3
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| url = http://www.springerlink.com/content/t0844nw244473143/
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| accessdate = 2006-08-06
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}}</ref><ref name="Neisser95" /><ref>{{Cite web
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|url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2218526&dopt=Abstract
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|title=Minnesota Study of Twins Reared Apart
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|accessmonthday=August 6 |accessyear=2006
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|date=[[October 12]], [[1990]]
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|author= Thomas J. Bouchard Jr.; David T. Lykken; Matthew McGue; Nancy L. Segal; Auke Tellegen
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|publisher=National Institutes of Health / Science, Oct 12, 1990 v250 n4978 p223(6)
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}}</ref>
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that is, depending on the study, a little less than half to substantially more than half of the variation in IQ among the children studied was due to variation in their genes. The remainder was thus due to environmental variation and measurement error. A heritability in the range of 0.4 to 0.8 implies that IQ is "substantially" heritable.
   
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The effect of restriction of range on IQ was examined by Matt McGue and colleagues, who write that "restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations ... IQ".<ref>http://dx.doi.org/10.1007/s10519-007-9142-7</ref> On the other hand, a 2003 study by Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman demonstrated that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes was close to zero.<ref>''Socioeconomic status modifies heritability of iq in young children'' Psychological Science, Volume 14 Issue 6 Page 623-628, November 2003</ref>
Environmental factors play a large role in determining IQ in situations where environmental conditions are variable. <!-- this is almost a tautology. Can we say "... in third world countries" to make it concrete? --> Proper childhood [[nutrition]] appears critical for [[cognitive development]]; [[malnutrition]] can lower IQ. Other research indicates environmental factors such as prenatal exposure to [[toxin]]s, duration of [[breastfeeding]], and [[micronutrient]] deficiency can affect IQ. However, in the developed world, none of these effects are sufficiently pronounced to be important. <!-- would rather say "... explain variance..." or something like that, but maybe it's too technical -->
 
   
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It is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. Surprisingly, the opposite occurs. Heritability measures in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood.<ref name="Plomin0103" /> The ''American Psychological Association's'' 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the white population the heritability of IQ is "around .75". The ''Minnesota Study of Twins Reared Apart'', a multiyear study of 100 sets of reared-apart twins which was started in 1979, concluded that about 70% of the variance in IQ was found to be associated with genetic variation. Some of the correlation of IQs of twins may be a result of the effect of the maternal environment before birth, shedding some light on why IQ correlation between twins reared apart is so robust.<ref>[http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=9242404&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus The heritability of IQ. ] Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):468-71.</ref>
In the developed world, there is some environmental effect on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, so that the cognitive ability of adults living in the prevailing conditions of the developed world is highly heritable.
 
   
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There are a number of points to consider when interpreting heritability:
Nearly all [[personality]] traits show that, contrary to expectations, environmental effects actually cause adoptive siblings raised in the same family to be as different as children raised in different families (Harris, 1998; Plomin & Daniels, 1987). Put another way, shared environmental variation for personality is zero, and all environmental effects are nonshared. Intelligence is actually an exception to this rule, at least among children. The IQs of adoptive siblings, who share no genetic relation but do share a common family environment, are correlated at .32. Despite attempts to isolate them, the factors that cause adoptive siblings to be similar have not been identified, though it could be related to parents choosing the type of children they will adopt. However, as explained below, shared family effects on IQ disappear after adolescence.
 
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*A high heritability does not mean that the environment has no effect on the development of a trait, or that learning is not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions.<ref name="Neisser95" />
  +
*A common error is to assume that because something is heritable it is necessarily unchangeable. This is wrong. Heritability does not imply immutability. As previously noted, heritable traits can depend on learning, and they may be subject to other environmental effects as well. The value of heritability can change if the distribution of environments (or genes) in the population is substantially altered. For example, an impoverished or suppressive environment could fail to support the development of a trait, and hence restrict individual variation. Differences in variation of heritability are found between developed and developing nations. This could affect estimates of heritability.<ref name="Neisser95" /> Another example is [[Phenylketonuria]] which previously caused mental retardation for everyone who had this genetic disorder. Today, this can be prevented by following a modified diet.
  +
*On the other hand, there can be effective environmental changes that do not change heritability at all. If the environment relevant to a given trait improves in a way that affects all members of the population equally, the mean value of the trait will rise without any change in its heritability (because the differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.<ref name="Neisser95" />
  +
*Even in developed nations, high heritability of a trait within a given group has no necessary implications for the source of a difference between groups.<ref>See: ''Ethnic Differences in Children's Intelligence Test Scores: Role of Economic Deprivation, Home Environment, and Maternal Characteristics''</ref><ref name="Neisser95" />
  +
===Environment===
  +
{{see also|Health and intelligence|environment and intelligence}}
  +
Environmental factors play a role in determining IQ. Proper childhood [[nutrition]] appears critical for [[cognitive development]]; [[malnutrition]] can lower IQ. Other research indicates environmental factors such as prenatal exposure to [[toxin]]s, duration of [[breastfeeding]], and [[micronutrient]] deficiency can affect IQ.
   
  +
It is well known that it is possible to increase one's IQ score by training, for example by regularly playing puzzle games, or strategy games like [[Chess]]. [[Music]]al training in childhood also increases IQ.<ref> Schellenberg, E. G. (2004). "Music lessons enhance IQ." Psychol Sci 15(8): 511-4. </ref> Recent studies have shown that training in using one's [[working memory]] may increase IQ.<ref> (Klingberg ''et al.'', 2002)</ref>
Active genotype-environment correlation, also called the "nature of nurture", is observed for IQ. This phenomenon is measured similarly to heritability; but instead of measuring variation in IQ due to genes, variation in environment due to genes is determined. One study found that 40% of variation in measures of home environment are accounted for by genetic variation. This suggests that the way human beings craft their environment is due in part to genetic influences.
 
   
===Development===
+
===Family environment===
  +
In the developed world, nearly all [[Wiktionary:personality|personality]] traits show that, contrary to expectations, environmental effects actually cause non-related children raised in the same family ("adoptive siblings") to be as different as children raised in different families (<ref>Harris, 1998</ref>; <ref name="Plomin0103"/>). There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, such that adoptive siblings are not more similar in IQ than strangers.<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9549239&dopt=Citation Genetic and environmental influences on adult intelligence and special mental abilities]. Human Biology, 70, 257–279. 1998</ref> For IQ, adoption studies show that, after adolescence, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: [[twin|monozygotic (identical) twins]] raised separately are highly similar in IQ (0.86), more so than [[Twin|dizygotic (fraternal) twins]] raised together (0.6) and much more than adoptive siblings (~0.0).<ref name="Plomin0103" />
  +
The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns'' (1995)<ref name="Neisser95" /> states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. Do differences between children's family environments (within the normal range) produce differences in their intelligence test performance? The problem here is to disentangle causation from correlation. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests. It also stated "We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence."<ref name="Neisser95" />
   
  +
A study of French children adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. The children came from poor backgrounds with IQs that initially averaged 77, putting them near retardation. Nine years later after adoption, they retook the I.Q. tests, and all of them did better. The amount they improved was directly related to the adopting family’s status. "Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."<ref>{{Cite web
It is reasonable to expect that genetic influences on traits like IQ should become less important as we gain experiences with age. Surprisingly, the opposite occurs. Heritability measures in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood.
 
  +
|url=http://www.nytimes.com/2006/07/23/magazine/23wwln_idealab.html?ei=5090&en=2c93740d624fe47f&ex=1311307200&partner=rssuserland&emc=rss&pagewanted=all
  +
|title=After the Bell Curve
  +
|accessmonthday=August 6 |accessyear=2006
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|date=[[July 23]], [[2006]]
  +
|author=David L. Kirp
  +
|publisher=New York Times Magazine
  +
}}</ref>
  +
On the other hand, the degree to which these increases persisted into adulthood are not clear from the study.
   
  +
Stoolmiller (1999)<ref name="Stoolmiller99">Stoolmiller, M. (1999). [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10414224&dopt=Citation Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior-genetic adoption studies.] Psychological Bulletin, 125, 392-409.</ref> found that the range restriction of family environments that goes with adoption, that adopting families tend to be more similar on for example socio-economic status than the general population, means that role of the shared family environment have been underestimated in previous studies. Corrections for range correction applied to adoption studies indicate that socio-economic status could account for as much as 50% of the variance in IQ.<ref name="Stoolmiller99" /> However, the effect of restriction of range on IQ for adoption studies was examined by Matt McGue and colleagues, who wrote that "restriction in range in parent disinhibitory psychopathology and family socio-economic status had no effect on adoptive-sibling correlations [in] IQ".<ref>http://dx.doi.org/10.1007/s10519-007-9142-7</ref>
Shared family effects also seem to disappear by adulthood. Adoption studies show that, after adolescence, adopted siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: [[twin|monozygotic (identical) twins]] raised separately are highly similar in IQ (0.86), more so than [[Twin|dizygotic (fraternal) twins]] raised together (0.6) and much more than adopted siblings (~0.0).
 
   
  +
Eric Turkheimer and colleagues (2003),<ref>Eric Turkheimer and colleagues (2003)</ref> not using an adoption study, included impoverished US families. Results demonstrated that the proportions of IQ variance attributable to genes and environment vary nonlinearly with socio-economic status. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.<ref>[http://www.blackwell-synergy.com/doi/abs/10.1046/j.0956-7976.2003.psci_1475.x?cookieSet=1 Socioeconomic status modifies heritability of iq in young children] Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman. Psychological Science 14 (6), 623–628. 2003</ref> They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.<ref>[http://www.connectforkids.org/node/516 New Thinking on Children, Poverty & IQ] November 10, 2003 Connect for Kids</ref>
Most of the IQ studies described above were conducted in developed countries, such as the [[United States]], [[Japan]], and [[Western Europe]]. Also, a few studies have been conducted in Moscow, East Germany, and India, and those studies produce similar results. Any such investigation is limited to describing the genetic and environmental variation found within the populations studied. This is a caveat of any heritability study.
 
   
===Mental retardation===
+
===Maternal (fetal) environment===
   
  +
A meta-analysis, by Devlin and colleagues in ''[[Nature (journal)|Nature]]'' (1997),<ref name="Devlin 97">[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9242404&dopt=Citation The heritability of IQ.] Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):417-8.</ref> of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (fetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%. They argue that the shared maternal environment may explain the striking correlation between the IQs of twins, especially those of adult twins that were reared apart {{Fact|date=July 2007}}. <!-- <ref name="Devlin 97" /> -->
About 75&ndash;80 percent of [[mental retardation]] is familial (runs in families), and 20&ndash;25 percent is due to organic problems, such as chromosomal abnormalities or brain damage.[http://www.isteve.com/2002_IQ_Supreme_Court_Death_Penalty.htm] Mild to severe mental retardation is a symptom of several hundred single-gene disorders and many chromosomal abnormalities, including small deletions. Based on twin studies, moderate to severe mental retardation does not appear to be familial, but mild mental retardation does. That is, the relatives of the moderate to severely mentally retarded have normal ranges of IQs, whereas the families of the mildly mentally retarded have IQs skewing lower.
 
   
  +
Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different than previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports.<ref name="Bouchard02">{{doi|10.1002/neu.10160}}</ref> They write that: <blockquote>Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al {{Fact|date=July 2007}} <!--. (1997a)<ref>Devlin et al. (1997a)</ref> --> conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950),{{Fact|date=July 2007}} <!-- <ref>Price (1950)</ref> --> in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion {{Fact|date=July 2007}} <!-- <ref>(Price, 1978)</ref> -->. Research subsequent to the 1978 review largely reinforces Price’s hypothesis {{Fact|date=July 2007}} <!-- <ref>Bryan, 1993</ref>; <ref>Macdonald et al., 1993</ref>; <ref>Hall and Lopez-Rangel, 1996</ref>; see also <ref>Martin et al., 1997, box 2</ref>; <ref>Machin, 1996</ref>).<ref name="Bouchard02" /> --></blockquote>
IQ score ranges (from DSM-IV):
 
* mild mental retardation: IQ 50&ndash;55 to 70; children require mild support; formally called "Educable Mentally Retarded".
 
* moderate retardation: IQ 35&ndash;40 to 50&ndash;55; children require moderate supervision and assistance; formally called "Trainable Mentally Retarded".
 
* severe mental retardation: IQ 20&ndash;25 to 35&ndash;40; can be taught basic life skills and simple tasks with supervision.
 
* profound mental retardation: IQ below 20&ndash;25; usually caused by a neurological condition; require constant care.
 
   
  +
===The Dickens and Flynn model===
The rate of mental retardation is higher among males than females, and higher among blacks than whites, according to a 1991 U.S. [[Centers for Disease Control and Prevention]] (CDC) study.[http://www.cdc.gov/MMWR/preview/mmwrhtml/00040928.htm]
 
  +
Dickens and Flynn (2001) <ref name="DickensFlynn2001">Dickens and Flynn (2001)</ref> postulate that the arguments regarding the disappearance of the shared family environment should apply equally well to groups separated in time. This is contradicted by the [[Flynn effect]]. Changes here have happened too quickly to be explained by genetic heritable adaptation. This [[paradox]] can be explained by observing that the measure "heritability" includes both a direct effect of the [[genotype]] on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.<ref name="DickensFlynn2001">William T. Dickens and James R. Flynn, [http://www.apa.org/journals/features/rev1082346.pdf Heritability Estimates Versus Large Environmental Effects:The IQ Paradox Resolved], ''Psychological Review'' 2001. Vol. 108, No. 2. 346-369.</ref><ref>William T. Dickens and James R. Flynn, "[http://www.brookings.edu/views/papers/dickens/20020205.pdf The IQ Paradox: Still Resolved]," ''Psychological Review'' 109, no. 4 (2002).</ref>
   
  +
==IQ and the brain==
By race, the overall rate was 16.6 per 1000 for blacks and 6.8 per 1000 for whites. Rates of mental retardation for black males, the group with the highest rates, were 1.7 times higher than black females, 2.4 times higher than white males, and 3.1 times higher than white females. <!--The following statement isn't clear or notable in its present form: Mild mental retardation is almost never diagnosed until a person enters elementary school, which critics claim lends strong support to the notion that IQ tests are racially biased.-->
 
  +
{{main|Neuroscience and intelligence}}
   
  +
In 2004, Richard Haier, professor of psychology in the Department of Pediatrics and colleagues at [[University of California, Irvine]] and the [[University of New Mexico]] used [[MRI]] to obtain structural images of the brain in 47 normal adults who also took standard IQ tests. The study demonstrated that general human intelligence appears to be based on the volume and location of [[gray matter]] tissue in the brain. Regional distribution of gray matter in humans is highly heritable. The study also demonstrated that, of the brain's gray matter, only about 6 percent appeared to be related to IQ.<ref>{{Cite web
Individuals with IQs below 70 have been exempted from the death penalty in the U.S. since 2002.[http://www.isteve.com/2002_IQ_Supreme_Court_Death_Penalty.htm]
 
  +
|url=http://today.uci.edu/news/release_detail.asp?key=1187
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|title=Human Intelligence Determined by Volume and Location of Gray Matter Tissue in Brain
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|accessmonthday=August 6 |accessyear=2006
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|date=July 19, 2004
  +
|author=Richard Haier
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|publisher=Brain Research Institute, UC Irvine College of Medicine
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}}</ref>
   
  +
Many different sources of information have converged on the view that the frontal lobes are critical for [[fluid intelligence]]. Patients with damage to the [[frontal lobe]] are impaired on fluid intelligence tests (Duncan et al 1995). The volume of frontal grey (Thompson et al 2001) and white matter (Schoenemann et al 2005) have also been associated with general intelligence. In addition, recent neuroimaging studies have limited this association to the lateral prefrontal cortex. Duncan and colleagues (2000) showed using [[Positron Emission Tomography]] that problem-solving tasks that correlated more highly with IQ also activate the lateral [[prefrontal cortex]]. More recently, Gray and colleagues (2003) used functional magnetic resonance imaging (fMRI) to show that those individuals that were more adept at resisting distraction on a demanding working memory task had both a higher IQ and increased prefrontal activity. For an extensive review of this topic, see Gray and Thompson (2004).<ref>{{Cite web
===IQ, education, and income===
 
  +
|url=http://www.loni.ucla.edu/~thompson/PDF/nrn0604-GrayThompson.pdf
  +
|title= Neurobiology of Intelligence: Science and Ethics
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|accessmonthday=August 6 |accessyear=2006
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|date=June 2004
  +
|author= Jeremy R. Gray, Psychology Department, Yale University, and Paul M. Thompson, Laboratory of Nero Imaging, Department of Neurology, University of California, Los Angeles School of Medicine
  +
|publisher=Nature Publishing Group, Volume 5
  +
}}</ref>
   
  +
A study involving 307 children (age between six to nineteen) measuring the size of brain structures using magnetic resonance imaging (MRI) and measuring verbal and non-verbal abilities has been conducted (Shaw et al 2006). The study has indicated that there is a relationship between IQ and the structure of the cortex—the characteristic change being the group with the superior IQ scores starts with thinner cortex in the early age then becomes thicker than average by the late teens.<ref>{{Cite web
Tambs ''et al.'' (1989) found that occupational status, educational attainment, and IQ are individually heritable; and further found that "genetic variance influencing educational attainment &hellip; contributed approximately one-fourth of the genetic variance for occupational status and nearly half the genetic variance for IQ". In a sample of US siblings, Rowe ''et al.'' (1997) report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.
 
  +
|url=http://www.bri.ucla.edu/bri_weekly/news_060330.asp
  +
|title=Scans Show Different Growth for Intelligent Brains
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|accessmonthday=August 6 |accessyear=2006
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|date=March 30, 2006
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|author=Nicholas Wade
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|publisher=Brain Research Institute, UCLA..
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}}</ref>
   
  +
Significant injuries isolated to one side of the brain, even those occurring at a young age, may not significantly affect IQ.<ref>Bava S, Ballantyne AO, Trauner DA. Disparity of verbal and performance IQ following early bilateral brain damage. J Int Neuropsychol Soc. 2004 Jan;10(1):82-90. [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=16175020&ordinalpos=75&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Pubmed link]</ref>
===Regression===
 
 
The heritability of IQ determines the extent to which the IQ of children will be similar to the IQ of parents. Because the heritability of IQ is less than 100%, the IQ of children tends to "regress" towards the mean IQ of the population. That is, high IQ parents tend to have children who are less bright than their parents, whereas low IQ parents tend to have children who are brighter than their parents. The effect can be quantified by the equation <math>\hat y = \bar x + h^2 \left ( \frac{\mbox{mom} + \mbox{dad}}{2} - \bar x \right)</math> where:
 
* <math>\hat y</math> is the predicted average IQ of Mom and Dad's children
 
* <math>\bar x</math> is the mean IQ of the population that Mom and Dad come from
 
* <math>h^2</math> is the heritability of IQ
 
Thus, if the heritability of IQ is 50%, a couple with an average IQ of 120 may have children that average around an IQ of 110, assuming that both parents come from a population with a median IQ of 100.
 
   
  +
Studies reach conflicting conclusions regarding the controversial idea that brain size correlates positively with IQ. Jensen and Reed (1993) claim no direct correlation exists in nonpathological subjects.<ref>Reed, T.E., & Jensen, A.R. 1993. Cranial capacity: new Caucasian data and comments on Rushton's claimed Mongoloid-Caucasoid brain-size differences. Intelligence, 17, 423-431</ref> A more recent meta-analysis suggests otherwise.<ref>McDaniel, M.A. (2005) Big-brained people are smarter: A meta-analysis of the relationship between ''in vivo'' brain volume and intelligence. ''Intelligence, 33'', 337-346. [http://www.people.vcu.edu/~mamcdani/Big-Brained%20article.pdf PDF]</ref>
==IQ and the Brain==
 
{{main|Neuroscience and intelligence}}
 
===Brain size and IQ===
 
Modern studies using [[MRI]] imaging have shown that brain size correlates with IQ by a factor of approximately .40 among adults (McDaniel, 2005). The correlation between brain size and IQ seems to hold for comparisons between and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found no within family correlation (Schoenemann et al. 2000). A [[Twin study|study on twins]] (Thompson ''et al.'', 2001) showed that frontal [[gray matter]] volume was correlated with ''g'' and highly heritable. A related study has reported that the correlation between brain size (reported to have a [[heritability]] of 0.85) and ''g'' is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al 2002).
 
   
  +
An alternative approach has sought to link differences in neural plasticity with intelligence (Garlick, 2002 <ref>Garlick, D. (2002). Understanding the nature of the general factor of intelligence: The role of individual differences in neural plasticity as an explanatory mechanism. Psychological Review, 109, 116-136. [http://www.apa.org/journals/releases/rev1091116.pdf]</ref>), and this view has recently received some empirical support (Shaw et al., 2006 <ref>Shaw, P., Greenstein, D., Lerch, J., Clasen, L., Lenroot, R., Gogtay, N., Evans, A., Rapoport, J., & Giedd, J. (2006). Intellectual ability and cortical development in children and adolescents. Nature, 440, 676-679.</ref>).
===Brain areas associated with IQ===
 
Many different sources of information have converged on the view that the frontal lobes are critical for fluid intelligence. Patients with damage to the frontal lobe are impaired on fluid intelligence tests (Duncan et al 1995). The volume of frontal grey (Thompson et al 2001) and white matter (Schoenemann et al 2005) have also been associated with intelligence. In addition, recent neuroimaging studies have limited this association to the lateral prefrontal cortex. Duncan and colleagues (2000) showed using Positron Emission Tomography that problem-solving tasks that correlated more highly with IQ also activate the lateral prefrontal cortex. More recently, Gray and colleagues (2003) used functional magnetic resonance imaging (fMRI) to show that those individuals that were more adept at resisting distraction on a demanding working memory task had both a higher IQ and increased prefrontal activity. For a review of this topic, see Gray and Thompson (2004).
 
   
 
==The Flynn effect==
 
==The Flynn effect==
 
{{main|Flynn effect}}
 
{{main|Flynn effect}}
   
Worldwide, IQ scores appear to be slowly rising, a trend known as the Flynn effect. However, tests are only renormalized occasionally to obtain mean scores of 100, for example WISC-R (1974), WISC-III (1991) and WISC-IV (2003). Hence it is difficult to compare IQ scores measured years apart.
+
The Flynn effect (aka the "Lynn-Flynn effect") was discovered by [[Richard Lynn]] in 1982, but is named after [[James R. Flynn]], a [[New Zealand]] based [[political scientist]]. Flynn showed that IQ scores worldwide appear to be slowly rising at a rate of around three IQ points per decade <ref>(Flynn, 1999)</ref>. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and [[heterosis]] <ref>{{citation |last=Mingroni |first=Michael A. |title=Resolving the IQ Paradox: Heterosis as a Cause of the Flynn Effect and Other Trends |date=2007 |pages=806-829}}</ref>. Tests are therefore renormalized occasionally to obtain mean scores of 100, for example [[Wechsler Intelligence Scale for Children|WISC]]-R (1974), WISC-III (1991) and WISC-IV (2003). Hence it is difficult to compare IQ scores measured years apart, unless this is compensated for.
   
  +
The Flynn effect may have ended in some developed nations starting in the mid 1990s. Teasdale & Owen (2005) "report intelligence test results from over 500,000 young [[Denmark|Danish]] men, tested between 1959 and 2004, showing that performance peaked in the late 1990s, and has since declined moderately to pre-1991 levels." They speculate that "a contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16–18 year olds."<ref>* Teasdale, Thomas W., and David R. Owen. (2005). "A long-term rise and recent decline in intelligence test performance: The Flynn Effect in reverse." ''Personality and Individual Differences.'' 39(4):837-843.</ref>
==IQ correlations==
 
 
===Race and IQ===
 
{{main|Race and intelligence}}
 
<!--Please read the race and intelligence article thoroughly before contributing to this section-->
 
   
  +
In 2004, Jon Martin Sundet of the [[University of Oslo]] and colleagues published an article documenting scores on intelligence tests given to [[Norway|Norwegian]] conscripts between the 1950s and 2002, showing that the increase in scores of general intelligence stopped after the mid-1990s and in numerical reasoning subtests, declined.<ref>{{Cite web
While the distributions of IQ scores among different racial-ethnic groups overlap considerably, groups differ in where their members cluster along the IQ scale. Some groups (e.g. East Asians and Jews) tend to cluster higher than whites, while other groups (e.g. blacks and Hispanics) tend to cluster lower than whites. Similar clustering occurs with related variables, such as school achievement, reaction time, and brain size. Many hypotheses have been proposed to explain racial-ethnic group differences in IQ. Neither test bias nor simple differences in socioeconomic status explain the IQ differences. The primary focus of the scientific debate is whether group differences are entirely caused by environmental factors or whether they also reflect a genetic component. The findings of this field are often thought to conflict with fundamental social philosophies, and have thus engendered a large controversy.
 
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|url=http://72.14.207.104/search?q=cache:sLUpdtOiKmoJ:www.missouri.edu/~aab2b3/LFE_GNXP/sdarticle.pdf+%2B%22Jon+Martin+Sundet%22+%2BIQ&hl=en&gl=us&ct=clnk&cd=9
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|title=The end of the Flynn Effect. A study of secular trends in mean intelligence scores of Norwegian conscripts during half a century.
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|accessmonthday=August 6 |accessyear=2006
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|date=
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|author=
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|publisher=
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}}</ref>
   
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==Group differences==
===Religiousness and IQ===
 
{{main|Religiousness and intelligence}}
 
   
  +
Among the most controversial issues related to the study of intelligence is the observation that intelligence measures such as IQ scores vary between populations. While there is little scholarly debate about the ''existence'' of some of these differences, the ''reasons'' remain highly controversial both within academia and in the public sphere.
Several studies show an inverse correlation between IQ and degree of religious belief. While almost all research indicates a negative correlation between intelligence and religiosity [http://kspark.kaist.ac.kr/Jesus/Intelligence%20&%20religion.htm], this remains a controversial point.
 
   
 
===Health and IQ===
 
===Health and IQ===
  +
{{main|Health and intelligence}}
   
  +
Persons with a higher IQ have generally lower adult [[morbidity]] and [[mortality]]. [[Post-Traumatic Stress Disorder]],<ref>{{Cite web
Persons with a higher IQ have generally lower adult morbidity and mortality. This may be because they better avoid injury and take better care of their own health. It also decreases the risk of [[Post-Traumatic Stress Disorder]], severe [[clinical depression|depression]], and [[schizophrenia]]. On the other hand, it increases the risk of [[Obsessive Compulsive Disorder]] [http://www.loni.ucla.edu/~thompson/PDF/GT_DM_5b.pdf].
 
  +
|url=http://archpsyc.ama-assn.org/cgi/content/abstract/63/11/1238
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|title= Intelligence and Other Predisposing Factors in Exposure to Trauma and Posttraumatic Stress Disorder. A Follow-up Study at Age 17 Years
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|accessmonthday=August 6 |accessyear=2006
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|date=11, November 2006
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|author= Naomi Breslau, PhD; Victoria C. Lucia, PhD; German F. Alvarado, MD, MPH
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|publisher= Arch Gen Psychiatry. 2006;63:1238-1245
  +
}}</ref> severe [[clinical depression|depression]], <ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1572949&dopt=Citation Effects of major depression on estimates of intelligence] Sackeim HA, Freeman J, McElhiney M, Coleman E, Prudic J, Devanand DP. J Clin Exp Neuropsychol. 1992 Mar;14(2):268-88.</ref><ref>[http://www.blackwell-synergy.com/doi/abs/10.1111/j.1440-1819.2006.01564.x Improvement of cognitive functioning in mood disorder patients with depressive symptomatic recovery during treatment: An exploratory analysis] LAURA MANDELLI, Psy. D, ALESSANDRO SERRETTI, md,1 CRISTINA COLOMBO, md, MARCELLO FLORITA, Psy. D, ALESSIA SANTORO, Psy. D, DAVID ROSSINI, MD, RAFFAELLA ZANARDI, MD AND ENRICO SMERALDI, MD. Psychiatry and Clinical Neurosciences Volume 60 Issue 5 Page 598 - October 2006</ref>
  +
and [[schizophrenia]] are less prevalent in higher IQ bands. High IQ has also been positively correlated with a lower risk for heart disease{{Citation needed|date=May 2010}}.
   
  +
A study of 11,282 individuals in Scotland who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, found an "inverse linear association" between childhood IQ scores and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained even after accounting for factors such as the child's socioeconomic background.<ref>{{Cite web
Research in Scotland has shown that a 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long [http://news.bbc.co.uk/hi/english/health/newsid_1260000/1260794.stm].
 
  +
|url=http://www.ajph.org/cgi/content/abstract/AJPH.2005.080168v1
  +
|title=Associations Between Childhood Intelligence and Hospital Admissions for Unintentional Injuries in Adulthood: The Aberdeen Children of the 1950s Cohort Study
  +
|accessmonthday=January 10 |accessyear=2007
  +
|date=2006
  +
|author= Debbie A. Lawlor, University of Bristol, Heather Clark, University of Aberdeen, David A. Leon, London School of Hygiene & Tropical Medicine
  +
|publisher=American Journal of Public Health, December 2006}}</ref>
  +
Research in Scotland has also shown that a 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.<ref>{{Cite web
  +
|url=http://www.bmj.com/cgi/content/abstract/322/7290/819
  +
|title=Longitudinal cohort study of childhood IQ and survival up to age 76
  +
|accessmonthday=August 6 |accessyear=2006
  +
|date=2001
  +
|author=Whalley and Deary
  +
|publisher=British Medical Journal 2001, 322:819-819
  +
}}</ref>
   
  +
A decrease in IQ has also been shown as an early predictor of late-onset [[Alzheimer's Disease]] and other forms of [[dementia]]. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia.<ref>{{Cite web
===Economic development and IQ===
 
  +
|url=http://www.jnnp.com/cgi/content/abstract/75/8/1100
A controversial book ''[[IQ and the Wealth of Nations]]'', claims to show that the wealth of a nation can in large part be explained by the average IQ score. This claim has been both disputed and supported in peer-reviewed papers. The data used has also been questioned.
 
  +
|title= Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype
  +
|accessmonthday=August 6 |accessyear=2006
  +
|date=2004
  +
|author=Cervilla et al
  +
|publisher=Psychiatry 2004;75:1100-1106.
  +
}}</ref> However, when diagnosing individuals with a higher level of cognitive ability, in this study those with IQ's of 120 or more,<ref>{{Cite web
  +
|url=http://laboratory-manager.advanceweb.com/common/editorial/editorial.aspx?CC=27318
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|title= More Sensitive Test Norms Better Predict Who Might Develop Alzheimer's Disease
  +
|accessmonthday=August 6 |accessyear=2006
  +
|date=
  +
|author= Dorene Rentz, Brigham and Women's Hospital's Department of Neurology and Harvard Medical School
  +
|publisher= Neuropsychology, published by the American Psychological Association
  +
}}</ref>
  +
patients should not be diagnosed from the standard norm but from an adjusted high-IQ norm that measured changes against the individual's higher ability level. In 2000, Whalley and colleagues published a paper in the journal ''Neurology'', which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.<ref>{{Cite web
  +
|url=http://www.neurology.org/cgi/content/abstract/55/10/1455
  +
|title= Childhood mental ability and dementia
  +
|accessmonthday=August 6 |accessyear=2006
  +
|date=2000
  +
|author=Whalley ''et al.''
  +
|publisher=Neurology 2000;55:1455-1459.
  +
}}</ref>
   
  +
Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the [[blood-brain barrier]] is less effective. Such impairment may sometimes be permanent, or may sometimes be partially or wholly compensated for by later growth. Several harmful factors may also combine, possibly causing greater impairment.
==Practical validity==
 
[[Image:Corr-example.png|thumb|Linear correlations between 1000 pairs of numbers. The data are graphed on the lower left and their correlation coefficients listed on the upper right. Each set of points correlates maximally with itself, as shown on the diagonal (all correlations = +1).]]
 
   
  +
Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring [[food fortification|fortification]] of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.<ref name="Olness">Olness, K. "[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&list_uids=12692458&dopt=Citation Effects on brain development leading to cognitive impairment: a worldwide epidemic]," ''Journal of Developmental and Behavioral Pediatrics'' 24, no. 2 (2003): 120&ndash;30.</ref>
Evidence for the practical validity of IQ comes from examining the [[correlation]] between IQ scores and life outcomes.
 
  +
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
 
  +
In terms of the effect of one's intelligence on health, high childhood IQ correlates with one's chance of becoming a vegetarian in adulthood ({{cite journal
|- bgcolor=#ccccff
 
  +
| last =Gale
|+ '''Economic and social correlates of IQ'''
 
  +
| first =CR
! Factors || Correlation
 
  +
| authorlink =
|- bgcolor=E9E8FF
 
  +
| coauthors =
| School grades and IQ || 0.5
 
  +
| title =IQ in childhood and vegetarianism in adulthood: 1970 British cohort study
|- bgcolor=#DFE0FF
 
  +
| journal =British Journal of Medicine
| Total years of education and IQ || 0.55
 
  +
| volume =334
|- bgcolor=E9E8FF
 
  +
| issue =7587
| IQ and parental socioeconomic status || 0.33
 
  +
| pages =245
|- bgcolor=#DFE0FF
 
  +
| date =
| Job performance and IQ || 0.54
 
  +
| url =
|- bgcolor=E9E8FF
 
  +
| doi =
| Negative social outcomes and IQ || &minus;0.2
 
  +
| id =
|- bgcolor=#DFE0FF
 
  +
| accessdate = }}), and inversely correlates with the chances of smoking ({{cite journal
| IQs of identical twins || 0.86
 
  +
| last =Taylor
|- bgcolor=E9E8FF
 
  +
| first =MD
| IQs of husband and wife || 0.4
 
  +
| authorlink =
|- bgcolor=#DFE0FF
 
  +
| coauthors =
| ''Heights'' of parent and child || 0.47
 
  +
| title =Childhood IQ and social factors on smoking behaviour, lung function and smoking-related outcomes in adulthood: linking the Scottish Mental Survey 1932 and the Midspan studies
|}
 
  +
| journal =British Journal of Health Psychology
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse; font-size: 95%;"
 
  +
| volume =10
|- bgcolor=#ccccff
 
  +
| issue =3
|+ '''Economic and social correlates of IQ'''
 
  +
| pages =399-401
  +
| date =
  +
| url =
  +
| doi =
  +
| id =
  +
| accessdate = }}), becoming obese, and having serious traumatic accidents in adulthood.
  +
  +
===Gender and IQ===
  +
{{main|Sex and intelligence}}
  +
  +
Most studies claim that despite sometimes significant differences in subtest scores, men and women have quite similar average IQ{{Fact|date=May 2007}}. Some studies claim that men outperform women on average by 3-4 IQ points.<ref>http://www.rlynn.co.uk/pages/publications.asp</ref><ref>{{cite journal | author=Stumpf, H. and Jackson, D. N. | title=Gender-related differences in cognitive abilities: evidence from a medical school admissions program | journal=Personality and Individual Differences | year=1994 | volume=17 | pages=335&ndash;344}}</ref> However, during a revision of some tests in the 1940's women outperformed men, which urged test makers to change the tests until men were able to score equally.<ref>Quinn McNemar, The Revision of the Stanford-Binet Scale, Houghton Mifflin Company, 1942</ref> Some studies claim that women perform better on tests of memory and verbal proficiency, while men perform better on tests of mathematical and spatial ability{{Fact|date=May 2007}}. Male scores display a higher [[variance]]: there are more men than women identified with both very high and very low IQs{{Fact|date=May 2007}}.
  +
  +
===Race and IQ===
  +
{{main|Race and intelligence}}
  +
<!--Please read the race and intelligence article thoroughly before contributing to this section-->
  +
  +
Much research has been devoted to the extent and potential causes of racial group differences in IQ.
  +
  +
==Positive correlations with IQ==
  +
{{globalize/USA}}
  +
<!--
  +
{| class="wikitable"
  +
|-
  +
|+ '''Economic and social correlates of IQ in the [[USA]]'''
 
! IQ || <75 || 75&ndash;90 || 90&ndash;110 || 110&ndash;125 || >125
 
! IQ || <75 || 75&ndash;90 || 90&ndash;110 || 110&ndash;125 || >125
  +
|-
|- bgcolor=E9E8FF
 
| US population distribution || 5 || 20 || 50 || 20 || 5
+
! U.S. population distribution || 5 || 20 || 50 || 20 || 5
  +
|-
|- bgcolor=#DFE0FF
 
 
| Married by age 30 || 72 || 81 || 81 || 72 || 67
 
| Married by age 30 || 72 || 81 || 81 || 72 || 67
  +
|-
|- bgcolor=E9E8FF
 
 
| Out of labor force more than 1 month out of year (men) || 22 || 19 || 15 || 14 || 10
 
| Out of labor force more than 1 month out of year (men) || 22 || 19 || 15 || 14 || 10
  +
|-
|- bgcolor=#DFE0FF
 
 
| Unemployed more than 1 month out of year (men) || 12 || 10 || 7 || 7 || 2
 
| Unemployed more than 1 month out of year (men) || 12 || 10 || 7 || 7 || 2
  +
|-
|- bgcolor=E9E8FF
 
 
| Divorced in 5 years || 21 || 22 || 23 || 15 || 9
 
| Divorced in 5 years || 21 || 22 || 23 || 15 || 9
  +
|-
|- bgcolor=#DFE0FF
 
| % of children w/ IQ in bottom decile (mothers) || 39 || 17 || 6 || 7 || -
+
| % of children w/ IQ in bottom decile (mothers) || 39 || 17 || 6 || 7 || < 1
  +
|-
|- bgcolor=E9E8FF
 
| Had an [[Illegitimacy|illegitimate]] baby (mothers) || 32 || 17 || 8 || 4 || 2
+
| Had a baby outside of marriage (mothers) || 32 || 17 || 8 || 4 || 2
  +
|-
|- bgcolor=#DFE0FF
 
| Lives in poverty || 30 || 16 || 6 || 3 || 2
+
| Lives in pooperty || 30 || 16 || 6 || 3 || 2
  +
|-
|- bgcolor=E9E8FF
 
| Ever incarcerated (men) || 7 || 7 || 3 || 1 || 0
+
| Ever incarcerated (men) || 7 || 7 || 3 || 1 || < 1
  +
|-
|- bgcolor=#DFE0FF
 
| Chronic welfare recipient (mothers) || 31 || 17 || 8 || 2 || 0
+
| Chronic welfare recipient (mothers) || 31 || 17 || 8 || 2 || < 1
  +
|-
|- bgcolor=E9E8FF
 
| High school dropout || 55 || 35 || 6 || 0.4 || 0
+
| High school dropout || 55 || 35 || 6 || 0.4 || < 0.4
  +
|-
|- bgcolor=#DFE0FF
 
| colspan="6" | Values are the percentage of each IQ sub-population fitting each descriptor. Compiled by Gottfredson (1997) from Herrnstein & Murray (1994) pp. 171, 158, 163, 174, 230, 180, 132, 194, 247&ndash;248, 194, 146 respectively.
+
| colspan="6" | Values are the percentage of each IQ sub-population, among [[white people|non-Hispanic whites]] only, fitting each descriptor. <ref>Compiled by Gottfredson (1997) from a U.S. study by Herrnstein & Murray (1994) pp. 171, 158, 163, 174, 230, 180, 132, 194, 247&ndash;248, 194, 146 respectively</ref>.
 
|}
 
|}
  +
-->
  +
While IQ is sometimes treated as an end unto itself, scholarly work on IQ focuses to a large extent on IQ's [[validity (psychometric)|validity]], that is, the degree to which IQ correlates with outcomes such as job performance, social pathologies, or academic achievement. Different IQ tests differ in their validity for various outcomes. Traditionally, correlation for IQ and outcomes is viewed as a means to also predict performance; however, because IQ is a known [[social artifact]], readers should distinguish between [[prediction]] in the [[hard sciences]] and the [[social sciences]].
   
  +
''Validity'' is the correlation between score (in this case cognitive ability, as measured, typically, by a paper-and-pencil test) and outcome (in this case job performance, as measured by a range of factors including supervisor ratings, promotions, training success, and tenure), and ranges between −1.0 (the score is perfectly wrong in predicting outcome) and 1.0 (the score perfectly predicts the outcome). See [[validity (psychometric)]].
Research shows that intelligence plays an important role in many valued life outcomes. In addition to academic success, intelligence correlates with job performance (see below), socioeconomic advancement (e.g., level of education, occupation, and income), and "social pathology" (e.g., adult criminality, poverty, unemployment, dependence on welfare, children outside of marriage). Recent work has demonstrated links between intelligence and health, longevity, and functional literacy. Correlations between ''[[g (factor)|g]]'' and life outcomes are pervasive, though IQ and [[happiness]] do not correlate. IQ and ''g'' correlate highly with school performance and job performance, less so with occupational prestige, moderately with income, and to a small degree with law-abidingness.
 
   
  +
Research shows that general intelligence plays an important role in many valued life outcomes. In addition to academic success, IQ correlates to some degree with job performance (see below), socioeconomic advancement (e.g., level of education, occupation, and income), and "social pathology" (e.g., adult criminality, poverty, unemployment, dependence on welfare, children outside of marriage). Recent work has demonstrated links between general intelligence and health, longevity, and functional literacy. Correlations between ''[[g (factor)|g]]'' and life outcomes are pervasive, though IQ does not correlate with subjective self-reports of happiness. IQ and ''g'' correlate highly with school performance and job performance, less so with occupational prestige, moderately with income, and to a small degree with law-abiding behaviour. IQ does not explain the inheritance of economic status and wealth.
General intelligence (in the literature typically called "cognitive ability") is the best predictor of job performance by the standard measure, validity. ''Validity'' is the correlation between score (in this case cognitive ability, as measured, typically, by a paper-and-pencil test) and outcome (in this case job performance, as measured by a range of factors including supervisor ratings, promotions, training success, and tenure), and ranges between &minus;1.0 (the score is perfectly wrong in predicting outcome) and 1.0 (the score perfectly predicts the outcome). See [[validity (psychometric)]]. The validity of cognitive ability for job performance tends to increase with job complexity and varies across different studies, ranging from 0.2 for unskilled jobs to 0.8 for the most complex jobs.
 
   
  +
=== Other tests ===
A large [[meta-analysis]] (Hunter and Hunter, 1984) which pooled validity results across many studies encompassing thousands of workers (32,124 for cognitive ability), reports that the validity of cognitive ability for entry-level jobs is 0.54, larger than any other measure including job tryout (0.44), experience (0.18), interview (0.14), age (&minus;0.01), education (0.10), and biographical inventory (0.37).
 
  +
One study found a correlation of .82 between ''g'' and [[SAT]] scores.[http://www.blackwell-synergy.com/links/doi/10.1111%2Fj.0956-797.2004.00687.x] Another correlation of .81 between ''g'' and [[GCSE]] scores.[http://www.sciencedirect.com/science/article/B6W4M-4JDN6DP-1/2/850d67264b9588a28059387bca359ff7]
   
  +
Correlations between IQ scores (general cognitive ability) and achievement test scores are reported to be .81 by Deary and colleagues, with the percentage of variance accounted for by general cognitive ability ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design"<ref>Ian J. Deary, Steve Strand, Pauline Smith and Cres Fernandes, Intelligence and educational achievement, Intelligence, Volume 35, Issue 1, January-February 2007, Pages 13-21.</ref>
Because higher test validity allows more accurate prediction of job performance, companies have a strong incentive to use cognitive ability tests to select and promote employees. IQ thus has high practical validity in economic terms. The [[utility]] of using one measure over another is proportional to the difference in their validities, all else equal. This is one economic reason why companies use job interviews (validity 0.14) rather than randomly selecting employees (validity 0.0).
 
   
  +
=== School performance ===
However, legal barriers, most prominently the 1971 United States Supreme Court decision ''[[Griggs v. Duke Power Co.]]'', have prevented American employers from directly using cognitive ability tests to select employees, despite the tests' high validity. This is largely based on that cognitive ability scores in selection adversely affects some minority groups, due to that different groups have different mean scores on tests of cognitive ability. However, cognitive ability tests are still used in some organizations. The U.S. military uses the [[Armed Services Vocational Aptitude Battery|Armed Forces Qualifying Test]] (AFQT), as higher scores correlate with significant increases in effectiveness of both individual soldiers and units,[http://www.rand.org/pubs/technical_reports/2005/RAND_TR193.pdf] [http://www.rand.org/pubs/monograph_reports/MR818/MR818.ch2.pdf] and [[Microsoft]] is known for using non-illegal tests that correlate with IQ tests as part of the interview process, weighing the results even more than experience in many cases.[http://www.highbeam.com/doc/1G1:18847742/Microsofts+big+advantage+-+hiring+only+the+supersmart%7eR%7e+(Company+Operations).html] [https://www.keepmedia.com/Auth.do?extId=10022&uri=/archive/forbes/2005/1031/045.html]
 
  +
The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns'' (1995)<ref name="Neisser95" /> Wherever it has been studied, children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. The correlation between IQ scores and grades is about .50. However, this means that they explain only 25% of the variance. Successful school learning depends on many personal characteristics other than intelligence, such as [[memory]], persistence, interest in school, and willingness to study.
   
  +
Correlations between IQ scores and total years of education are about .55, implying that differences in psychometric intelligence account for about 30% of the outcome variance. Many occupations can only be entered through professional schools which base their admissions at least partly on test scores: the MCAT, the GMAT, the GRE, the DAT, the LSAT, etc. Individual scores on admission-related tests such as these are certainly correlated with scores on tests of intelligence. It is partly because intelligence test scores predict years of education that they also predict occupational status, and income to a smaller extent.
Some researchers have echoed the popular claim that "in economic terms it appears that the IQ score measures something with decreasing marginal value. It is important to have enough of it, but having lots and lots does not buy you that much." (Detterman and Daniel, 1989)[http://www.americanscientist.org/template/AssetDetail/assetid/24538/page/4]
 
   
  +
=== Job performance ===
However, some studies suggest IQ continues to confer large benefits even at very high levels. Ability and performance for jobs are linearly related, such that at all IQ levels, an increase in IQ translates into a concomitant increase in performance (Coward and Sackett, 1990). In an analysis of hundreds of siblings, it was found that IQ has a substantial effect on income independently of family background (Murray, 1998).
 
   
  +
According to Schmidt and Hunter, "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability."<ref name="Schmidt98">Schmidt, F. L. and Hunter, J. E. (1998). The validity and utility of selection methods in psychology: practical and theoretical implications of 85 years of research findings. Psychological Bulletin, 124, 262–274.</ref> The validity depends on the type of job and varies across different studies, ranging from 0.2 to 0.6 <ref name="Hunter84">Hunter, J. E. and Hunter, R. F. (1984). Validity and utility of alternative predictors of job performance. Psychological Bulletin, 96, 72–98.</ref>. However IQ mostly correlates with cognitive ability only if IQ scores are below average and this rule has many (about 30 %) exceptions for people with average and higher IQ scores <ref name="DiazAsper1">Diaz-Asper CM, Schretlen DJ, Pearlson GD. How well does IQ predict neuropsychological test performance in normal adults? J Int Neuropsychol Soc. 2004 Jan;10(1):82-90. [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=14751010&ordinalpos=22&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum PubMed link] </ref>. Also, IQ is related to the "academic tasks" (auditory and linguistic measures, memory tasks, academic achievement levels) and much less related to tasks where even precise hand work ("motor functions") are required <ref>Warner MH, Ernst J, Townes BD, Peel J, Preston M Relationships between IQ and neuropsychological measures in neuropsychiatric populations: within-laboratory and cross-cultural replications using WAIS and WAIS-R. J Clin Exp Neuropsychol. 1987 Oct;9(5):545-62. [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=3667899&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus PubMed link] </ref>
Other studies question the real-world importance of whatever is measured with IQ tests, especially for differences in accumulated [[wealth]] and general [[economic inequality]] in a nation. IQ correlates highly with school performance but the correlations decrease the closer one gets to real-world outcomes, like with job performance, and still lower with income. It explains less than one sixth of the income variance [http://www.lrainc.com/swtaboo/taboos/apa_01.html]. Even for school grades, other factors explain most the variance. Regarding economic inequality, one study found that if we could magically give everyone identical IQs, we would still see 90 to 95 percent of the inequality we see today. [http://home.att.net/~Resurgence/L-IQpredicts.htm]. Another recent study (2002) found that wealth, race, and schooling are important to the inheritance of economic status, but IQ is not a major contributor and the genetic transmission of IQ is even less important [http://www.umass.edu/preferen/gintis/intergen.pdf]. Some argue that IQ scores are used as an excuse for not trying to reduce poverty or otherwise improve living standards for all. Claimed low intelligence has historically been used to justify the [[feudal system]] and unequal treatment of women (but note that many studies find identical average IQs among men and women; see [[sex and intelligence]]). In contrast, others claim that the refusal of high-IQ elites to take IQ seriously as a cause of inequality is itself immoral.[http://www.isteve.com/How_to_Help_the_Left_Half_of_the_Bell_Curve.htm]
 
   
  +
A [[meta-analysis]] (Hunter and Hunter, 1984)<ref name="Hunter84" /> which pooled validity results across many studies encompassing thousands of workers (32,124 for cognitive ability), reports that the validity of cognitive ability for entry-level jobs is 0.54, larger than any other measure including job try-out (0.44), experience (0.18), interview (0.14), age (−0.01), education (0.10), and biographical inventory (0.37). This implies that, across a wide range of occupations, intelligence test performance accounts for some 29% of the variance in job performance.
==Use of IQ in the United States legal system==
 
   
  +
According to Marley Watkins and colleagues, IQ is a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.<ref>Marley W. Watkins, Pui-Wa Lei and Gary L. Canivez, Psychometric intelligence and achievement: A cross-lagged panel analysis, Intelligence, Volume 35, Issue 1, January-February 2007, Pages 59-68.</ref> Treena Eileen Rohde and Lee Anne Thompson write that general cognitive ability but not specific ability scores predict academic achievement, with the exception that processing speed and spatial ability predict performance on the SAT math beyond the effect of general cognitive ability.<ref>Treena Eileen Rohde and Lee Anne Thompson, Predicting academic achievement with cognitive ability, Intelligence, Volume 35, Issue 1, January-February 2007, Pages 83-92.</ref>
The Supreme Court of the United States has also validated the use of IQ results during the sentencing phase of some criminal proceedings. The Supreme Court case of ''Atkins v. Virginia'', decided [[June 20]] [[2002]], [http://supct.law.cornell.edu/supct/html/00-8452.ZO.html] held that executions of mentally retarded criminals are "cruel and unusual punishments" prohibited by the Eighth Amendment. In ''Atkins'' the court stated that
 
   
  +
The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns'' (1995)<ref name="Neisser95" /> states that other individual characteristics such as interpersonal skills, aspects of personality, etc., are probably of equal or greater importance, but at this point we do not have equally reliable instruments to measure them.<ref name="Neisser95" />
:"&hellip;[I]t appears that even among those States that regularly execute offenders and that have no prohibition with regard to the mentally retarded, only five have executed offenders possessing a known IQ less than 70 since we decided ''Penry''. The practice, therefore, has become truly unusual, and it is fair to say that a national consensus has developed against it."
 
   
  +
=== Income ===
In overturning the Virginia Supreme Court's holding, the ''Atkins'' opinion stated that petitioner's IQ result of 59 was a factor making the imposition of capital punishment a violation of his eighth amendment rights. In the opinion's notes the court provided some of the facts relied upon when reaching their decision
 
  +
<!--
  +
{| class="wikitable"
  +
|-
  +
|+ '''Relation between IQ and earnings in the U.S.'''
  +
! IQ || <75 || 75&ndash;90 || 90&ndash;110 || 110&ndash;125 || >125
  +
|-
  +
| Age 18 || 2,000 || 5,000 || 8,000 || 8,000 || 21,000
  +
|-
  +
| Age 26 || 3,000 || 10,000 || 16,000 || 20,000 || 42,000
  +
|-
  +
| Age 32 || 5,000 || 12,400 || 20,000 || 27,000 || 48,000
  +
|-
  +
| colspan="6" | Values are the average earnings (1993 US Dollars) of each IQ sub-population.<ref>Murray, C. (1997). IQ and economic success. Public Interest, 128, 21–35.</ref>
  +
|}
  +
-->
  +
Some researchers claim that "in economic terms it appears that the IQ score measures something with decreasing marginal value. It is important to have enough of it, but having lots and lots does not buy you that much."<ref>Detterman and Daniel, 1989.</ref><ref>{{Cite web
  +
|url=http://www.americanscientist.org/template/AssetDetail/assetid/24538/page/4
  +
|title=The Role of Intelligence in Modern Society
  +
|pages=4 (Nonlinearities in Intelligence)
  +
|accessmonthday=August 6|accessyear=2006
  +
|datemonth=July|dateyear=1995
  +
|author=Earl Hunt
  +
|publisher=American Scientist
  +
}}</ref>
   
  +
Other studies show that ability and performance for jobs are linearly related, such that at all IQ levels, an increase in IQ translates into a concomitant increase in performance <ref>Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. ''Journal of Applied Psychology,'' 75:297–300.</ref>. Charles Murray, coauthor of ''[[The Bell Curve]]'', found that IQ has a substantial effect on income independently of family background <ref>Murray, Charles (1998). Income Inequality and IQ, AEI Press [http://www.aei.org/docLib/20040302_book443.pdf PDF]</ref>.
:"At the sentencing phase, Dr. Nelson testified: "Atkins' full scale IQ is 59. Compared to the population at large, that means less than one percentile&hellip;. Mental retardation is a relatively rare thing. It's about one percent of the population." App. 274. According to Dr. Nelson, Atkins' IQ score "would automatically qualify for Social Security disability income." Id., at 280. Dr. Nelson also indicated that of the over 40 capital defendants that he had evaluated, Atkins was only the second individual who met the criteria for mental retardation. Id., at 310. He testified that, in his opinion, Atkins' limited intellect had been a consistent feature throughout his life, and that his IQ score of 59 is not an "aberration, malingered result, or invalid test score." Id., at 308."
 
   
  +
The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns'' (1995)<ref name="Neisser95" /> states that IQ scores account for about one-fourth of the social status variance and one-sixth of the income variance. Statistical controls for parental SES eliminate about a quarter of this predictive power. Psychometric intelligence appears as only one of a great many factors that influence social outcomes.<ref name="Neisser95" />
==Validity and ''g''-loading of specific tests==
 
   
  +
One reason why some studies claim that IQ only accounts for a sixth of the variation in income is because many studies are based on young adults (many of whom have not yet completed their education). On pg 568 of [[The g factor]], [[Arthur Jensen]] claims that although the correlation between IQ and income averages a moderate 0.4 (one sixth or 16% of the variance), the relationship increases with age, and peaks at middle age when people have reached their maximum career potential. In the book, a [[Question of Intelligence]], [[Daniel Seligman]] cites an IQ income correlation of 0.5 (25% of the variance).
While IQ is sometimes treated as an end unto itself, scholarly work on IQ focuses to a large extent on IQ's [[validity (psychometric)|validity]], that is, the degree to which IQ predicts outcomes such as job performance, social pathologies, or academic achievement. Different IQ tests differ in their validity for various outcomes.
 
   
  +
A 2002 study<ref>[http://www.ingentaconnect.com/content/aea/jep/2002/00000016/00000003/art00001 The Inheritance of Inequality] Bowles, Samuel; Gintis, Herbert. The Journal of Economic Perspectives. Volume 16, Number 3, 1 August 2002, pp. 3-30(28)</ref> further examined the impact of non-IQ factors on income and concluded that an offspring's inherited wealth, race, and schooling are more important as factors in determining income than IQ.
Tests also differ in their [[General intelligence factor|''g'']]-loading, which is the degree to which the test score reflects general mental ability rather than a specific skill or "group factor" such as verbal ability, spatial visualization, or mathematical reasoning). ''g''-loading and validity are related in the sense that most IQ tests derive their validity mostly or entirely from the degree to which they measure ''g'' (Jensen 1998).
 
   
  +
=== Other correlations with IQ ===
==Social construct==
 
  +
<!--
  +
{| class="wikitable"
  +
|-
  +
|+ '''Economic and social correlates of IQ'''
  +
! Factors || Correlation
  +
|-
  +
| School grades and IQ || 0.5
  +
|-
  +
| Total years of education and IQ || 0.55
  +
|-
  +
| IQ and parental socioeconomic status || 0.33
  +
|-
  +
| Job performance and IQ || 0.54
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|-
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| Negative social outcomes and IQ || −0.2
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|-
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| IQs of identical twins || 0.86
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|-
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| IQs of husband and wife || 0.4
  +
|-
  +
| GDP per capita and average national IQ || 0.7
  +
|}
  +
-->
  +
In addition, IQ and its correlation to health, [[violent crime]], [[gross state product]], and government effectiveness are the subject of a 2006 paper in the publication ''Intelligence''. The paper breaks down IQ averages by U.S. states using the federal government's [[National Assessment of Educational Progress]] math and reading test scores as a source.<ref>{{Cite web
  +
|url=http://www.people.vcu.edu/~mamcdani/Publications/McDaniel%20(2006)%20Estimating%20state%20IQ.pdf
  +
|title=Estimating state IQ: Measurement challenges and preliminary correlates
  +
|accessmonthday= |accessyear=
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|date=accepted for publication [[August]] [[2006]]
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|author= Michael A. McDaniel, Virginia Commonwealth University
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|publisher=Intelligence
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|format=PDF
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}}</ref>
   
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There is a correlation of -.19 between IQ scores and number of juvenile offences in a large Danish sample; with social class controlled, the correlation dropped to -. 17. Similarly, the correlations for most "negative outcome" variables are typically smaller than .20, which means that test scores are associated with less than 4% of their total variance. It is important to realize that the causal links between psychometric ability and social outcomes may be indirect. Children who are unsuccessful in - and hence alienated from - school may be more likely to engage in delinquent behaviours for that very reason, compared to other children who enjoy school and are doing well.<ref name="Neisser95" />
Some maintain that IQ is a [[social construct]] invented by the privileged classes, used to maintain their privilege. Others maintain that intelligence, measured by IQ or ''g'', reflects a real ability, is a useful tool in performing life tasks and has a biological reality.
 
   
  +
IQ is also associated with [[Health and intelligence#Association with other diseases|certain diseases]].
The social-construct and real-ability interpretations for IQ differences can be distinguished because they make opposite predictions about what would happen if people were given equal opportunities. The social explanation predicts that equal treatment will eliminate differences, while the real-ability explanation predicts that equal treatment will accentuate differences. Evidence for both outcomes exists. Achievement gaps persist in socioeconomically advantaged, integrated, liberal, suburban school districts in the United States (see Noguera, 2001). Test-score gaps tend to be larger at higher socioeconomic levels (Gottfredson, 2003). Some studies have reported a narrowing of score gaps over time.
 
   
  +
The book ''[[IQ and the Wealth of Nations]]'' claims to show that the [[GDP]]/person of a nation can in large part be explained by the average IQ score of its citizens. This claim has been both disputed and supported in peer-reviewed papers. The data used have also been questioned.
The reduction of intelligence to a single score seems extreme and wrong to many people. Opponents argue that it is much more useful to know a person's strengths and weaknesses than to know their IQ score. Such opponents often cite the example of two people with the same overall IQ score but very different ability profiles. As measured by IQ tests, most people have highly balanced ability profiles, with differences in subscores being greater among the more intelligent.
 
   
  +
Tambs ''et al.'' (1989)<ref>Tambs K, Sundet JM, Magnus P, Berg K. "Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: a study of twins." Behav Genet. 1989 Mar;19(2):209&ndash;22. PMID 2719624.</ref> found that occupational status, educational attainment, and IQ are individually heritable; and further found that "genetic variance influencing educational attainment … contributed approximately one-fourth of the genetic variance for occupational status and nearly half the genetic variance for IQ". In a sample of U.S. siblings, Rowe ''et al.'' (1997)<ref>Rowe, D. C., W. J. Vesterdal, and J. L. Rodgers, "The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations," University of Arizona, 1997</ref> report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.
The creators of IQ testing did not intend for the tests to gauge a person's worth, and in many (or, as some people suggest, all) situations, IQ may have little relevance.
 
 
===The Mismeasure of Man===
 
   
  +
Some argue that IQ scores are used as an excuse for not trying to reduce poverty or otherwise improve living standards for all. Claimed low intelligence has historically been used to justify the [[feudal system]] and unequal treatment of women (but note that many studies find identical average IQs among men and women; see [[sex and intelligence]]). In contrast, others claim that the refusal of "high-IQ elites" to take IQ seriously as a cause of inequality is itself immoral.<ref>{{Cite web
Some scientists dispute [[psychometrics]] entirely. In ''[[The Mismeasure of Man]]'', Professor [[Stephen Jay Gould]] argues that intelligence tests are based on faulty assumptions and shows their history of being used as the basis for [[scientific racism]]. He writes:
 
  +
|url=http://www.isteve.com/How_to_Help_the_Left_Half_of_the_Bell_Curve.htm
  +
|title=How to Help the Left Half of the Bell Curve
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|accessmonthday=August 6 |accessyear=2006
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|datemonth=July|dateyear=2000
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|author=Steve Sailer
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|publisher=VDARE.com
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}}</ref>
   
  +
==Public policy==
:&hellip;the abstraction of intelligence as a single entity, its location within the brain, its quantification as one number for each individual, and the use of these numbers to rank people in a single series of worthiness, invariably to find that oppressed and disadvantaged groups&mdash;races, classes, or sexes&mdash;are innately inferior and deserve their status. (pp. 24&ndash;25)
 
  +
{{main|Intelligence and public policy}}
   
  +
In the [[United States]], certain [[public policy|public policies]] and [[law]]s regarding military service,<ref>{{Cite web
He spends much of the book debunking the concept of IQ, including a historical discussion of how the IQ tests were created and a technical discussion of why ''g'' is simply a mathematical artifact. Later editions of the book include criticism of ''[[The Bell Curve]]''.
 
  +
|url=http://www.rand.org/pubs/technical_reports/2005/RAND_TR193.pdf
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|title=RAND_TR193.pdf
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|format=PDF
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}}, {{Cite web
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|url=http://www.rand.org/pubs/monograph_reports/MR818/MR818.ch2.pdf
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|title=MR818.ch2.pdf
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}}</ref>
  +
education, public benefits,<ref>{{Cite web|url=http://www.ssa.gov/disability/professionals/bluebook/12.00-MentalDisorders-Adult.htm
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|title=SOCIAL SECURITY ADMINISTRATION
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|accessmonthday=
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|date=
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crime,<ref>{{Cite web
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|url=http://www.isteve.com/2002_IQ_Supreme_Court_Death_Penalty.htm
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|title=IQ Defenders Feel Vindicated by Supreme Court
  +
|accessmonthday=August 6 |accessyear=2006
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|date=Steve Sailer
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|author=June 24, 2002
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|publisher=UPI
  +
}}</ref>and employment incorporate an individual's IQ or similar measurements into their decisions. However, in 1971 the U.S. Supreme Court had banned the use of IQ tests in employment, except in very rare cases<ref>Nicholas Lemann. The IQ Meritocracy. Time 100 [http://www.time.com/time/time100/scientist/other/iq.html link]</ref>. Internationally, certain public policies, such as improving nutrition and prohibiting [[neurotoxins]], have as one of their goals raising or preventing a decline in intelligence.
   
  +
==Criticism and views==
[[Arthur Jensen]], Professor of Educational Psychology, University of California, Berkeley, responds to Gould's criticisms in a paper titled ''The Debunking of Scientific Fossils and Straw Persons''.[http://www.mugu.com/cgi-bin/Upstream/jensen-gould-fossils]
 
  +
===Binet===
  +
[[Alfred Binet]] did not believe that IQ test scales qualified to measure intelligence. He neither invented the term "intelligence quotient" nor supported its numerical expression. He stated:
   
  +
:The scale, properly speaking, does not permit the measure of intelligence, because intellectual qualities are not superposable, and therefore cannot be measured as linear surfaces are measured. (Binet 1905)
===The view of the American Psychological Association===
 
In response to the controversy surrounding ''The Bell Curve'', the [[American Psychological Association]]'s Board of Scientific Affairs established a task force to write a consensus statement on the state of intelligence research which could be used by all sides as a basis for discussion. The full text of the report is available at a third-party website. [http://www.lrainc.com/swtaboo/taboos/apa_01.html]
 
   
  +
Binet had designed the Binet-Simon intelligence scale in order to identify students who needed special help in coping with the school curriculum. He argued that with proper remedial education programs, most students regardless of background could catch up and perform quite well in school. He did not believe that intelligence was a measurable fixed entity.
The findings of the task force state that IQ scores ''do'' have high predictive validity for individual (but not necessarily population) differences in school achievement. They confirm the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They agree that individual (again, not necessarily population) differences in intelligence are substantially influenced by genetics.
 
   
  +
Binet cautioned:
They state there is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition. They agree that there are no significant differences between the average IQ scores of males and females. The task force agrees that large differences do exist between the average IQ scores of blacks and whites, and that these differences cannot be attributed to biases in test construction. While they admit there is no empirical evidence supporting it, the APA task force suggests that explanations based on social status and cultural differences may be possible. Regarding genetic causes, they noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.
 
   
  +
:Some recent thinkers seem to have given their moral support to these deplorable verdicts by affirming that an individual's intelligence is a fixed quantity, a quantity that cannot be increased. We must protest and react against this brutal pessimism; we must try to demonstrate that it is founded on nothing.<ref>Rawat, R. [http://www.rso.cornell.edu/scitech/archive/95sum/bell.html The Return of Determinism?]</ref>
The APA journal that published the statement, ''[[American Psychologist]]'', subsequently published eleven critical responses in January 1997, most arguing that the report failed to examine adequately the evidence for partly-genetic explanations.
 
   
  +
===The Mismeasure of Man===
The report was published in 1995 and thus does not include a decade of recent research.
 
  +
Some scientists dispute [[psychometrics]] entirely. In ''[[The Mismeasure of Man]]'' professor [[Stephen Jay Gould]] argued that intelligence tests were based on faulty assumptions and showed their history of being used as the basis for [[scientific racism]]. He wrote:
   
  +
:…the abstraction of intelligence as a single entity, its location within the brain, its quantification as one number for each individual, and the use of these numbers to rank people in a single series of worthiness, invariably to find that oppressed and disadvantaged groups—races, classes, or sexes—are innately inferior and deserve their status. (pp. 24–25)
==Improving IQ==
 
While a large amount of one's IQ is predetermined by genetic factors, the environment can play a role as well. IQ can be improved to a certain extent through reading and application. Improvement in diet and regular exercise can help certain cognitive functions, and getting more sleep may help as well. Depression and stress reduce IQ somewhat, so removal of these factors might also help.
 
   
  +
He spent much of the book criticizing the concept of IQ, including a historical discussion of how the IQ tests were created and a technical discussion of why ''g'' is simply a mathematical artifact. Later editions of the book included criticism of ''[[The Bell Curve]]''.
Drugs designed to improve cognitive function, and sometimes IQ scores are called [[nootropic]]s.
 
   
  +
Gould does not dispute the stability of test scores, nor the fact that they predict certain forms of achievement. He does argue, however, that to base a concept of intelligence on these test scores alone is to ignore many important aspects of mental ability.
Working memory training, an experimental treatment which has according to one study by Klingberg et al, improved raw scores substantially on Ravens progressive matrices and Ravens advanced progressive matrices, both IQ tests. It has also been claimed in some studies that [[neurofeedback]] can increase IQ. However, some would argue that these studies should not necessarily be interpreted as proof that neurofeedback can increase IQ as (a) they don't have a double blind component and (b) it is unknown whether their effects would apply to persons without ADHD, as most of these studies were performed on persons with ADHD. It is possible that the increase in IQ was just a result of better concentration in the subjects.
 
   
  +
=== Relation between IQ and intelligence ===
A recent scientific article on the concept of cognitive reserve included an argument that education and application of the mind can substantially increase IQ.
 
  +
{{see also|Intelligence}}
  +
Several other ways of measuring intelligence have been proposed. [[Daniel Schacter]], [[Daniel Gilbert (psychologist)|Daniel Gilbert]], and others have moved beyond general intelligence and IQ as the sole means to describe intelligence.<ref>''[http://select.nytimes.com/2007/09/14/opinion/14brooks.html The Waning of I.Q.]'' by [[David Brooks]], [[The New York Times]]</ref>
   
  +
=== Test bias ===
The "[[Mozart effect]]" is the claimed ability of certain musics to enhance intelligence, especially spatial reasoning. However, this effect is not universally accepted. Musical education, as opposed to appreciation, has been shown a number of times to marginally increase IQ in children; however, there is sparsity of information on whether such an effect might apply to adults.
 
  +
{{Seealso|Stereotype threat}}
  +
The [[American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns'' (1995)<ref name="Neisser95" /> states that that IQ tests as predictors of social achievement are not biased against people of African descent since they predict future performance, such as school achievement, similarly to the way they predict future performance for European descent.<ref name="Neisser95" />
   
  +
However, IQ tests may well be biased when used in other situations. A 2005 study finds some evidence that the WAIS-R is not culture-fair for Mexican Americans.<ref>[http://asm.sagepub.com/cgi/content/abstract/12/3/303 Culture-Fair Cognitive Ability Assessment] Steven P. Verney Assessment, Vol. 12, No. 3, 303-319 (2005)</ref> Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.<ref>[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15742541&dopt=Abstract Cross-cultural effects on IQ test performance: a review and preliminary normative indications on WAIS-III test performance.] Shuttleworth-Edwards AB, Kemp RD, Rust AL, Muirhead JG, Hartman NP, Radloff SE. J Clin Exp Neuropsychol. 2004 Oct;26(7):903-20.</ref><ref>[http://www.blackwell-synergy.com/doi/abs/10.1111/j.1468-2389.2006.00346.x Case for Non-Biased Intelligence Testing Against Black Africans Has Not Been Made: A Comment on Rushton, Skuy, and Bons (2004)] 1*, Leah K. Hamilton1, Betty R. Onyura1 and Andrew S. Winston International Journal of Selection and Assessment Volume 14 Issue 3 Page 278 - September 2006</ref> Standard intelligence tests, such as the Stanford-Binet, are often inappropriate for children with [[autism]]; the alternative of using developmental or adaptive skills measures are relatively poor measures of intelligence in autistic children, and have resulted in incorrect claims that a majority of children with autism are mentally retarded.<ref>{{cite journal |author= Edelson, MG |date=2006 |title= Are the majority of children with autism mentally retarded? a systematic evaluation of the data |journal= Focus Autism Other Dev Disabl |volume=21 |issue=2 |pages=66–83 |url=http://www.willamette.edu/dept/comm/reprint/edelson/ |accessdate=2007-04-15}}</ref>
The levels of a variety of chemicals in the brain, such as chlorine, have been shown to relate to intelligence in a variety of ways. It is possible that by adjusting diet, these could be substantially changed.
 
   
  +
===Outdated methodology===
Future possibilities for improving the skills IQ tests measure include stem cells treatment, genetic modification, better education based on neurological and cognitive discoveries, better nootropics, etc. [http://www.newscientist.com/channel/being-human/mg18625011.900.]
 
  +
A 2006 paper argues that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s."<ref>[http://users.fmg.uva.nl/dborsboom/papers.htm The attack of the psychometricians]. Denny Borsboom. Psychometrika Vol. 71, No. 3, 425–440. September 2006.</ref>It also claims that some of the most influential recent studies on group differences in intelligence, in order to show that the tests are unbiased, use outdated methodology.
   
  +
=== The view of the American Psychological Association ===
==Controversy==
 
   
  +
In response to the controversy surrounding ''[[The Bell Curve]]'', the [[American Psychological Association]]'s Board of Scientific Affairs established a task force in 1995 to write a consensus statement on the state of intelligence research which could be used by all sides as a basis for discussion. The [http://www.gifted.uconn.edu/siegle/research/Correlation/Intelligence.pdf full text] of the report is available through several websites.<ref name="Neisser95" />
See article on [[IQ test controversy]].
 
   
  +
In this paper the representatives of the association regret that IQ-related works are frequently written with a view to their political consequences: "research findings were often assessed not so much on their merits or their scientific standing as on their supposed political implications".
==End material==
 
===See also===
 
   
  +
The task force concluded that IQ scores do have high predictive validity for individual differences in school achievement. They confirm the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They agree that individual (but specifically not population) differences in intelligence are substantially influenced by genetics.
   
  +
They state there is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition. They agree that there are no significant differences between the average IQ scores of males and females. The task force agrees that large differences do exist between the average IQ scores of blacks and whites, and that these differences cannot be attributed to biases in test construction. The task force suggests that explanations based on social status and cultural differences are possible, and that environmental factors have raised mean test scores in many populations. Regarding genetic causes, they noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.
  +
  +
The APA journal that published the statement, ''[[American Psychologist]]'', subsequently published eleven critical responses in January 1997, several of them arguing that the report failed to examine adequately the evidence for partly-genetic explanations.
  +
  +
  +
  +
==High IQ societies==
  +
{{main| High IQ society}}
  +
A high IQ society is an organization that limits membership to people who are within a certain high percentile of IQ test results. The most well-known is [[Mensa International]], which requires members to score in the top 2% of a [[standardized IQ test]].
  +
  +
==Reference charts==
  +
{{main|IQ reference chart}}
  +
IQ reference charts are tables suggested by psychologists to divide intelligence ranges in various categories.
   
 
==See also==
 
==See also==
  +
{{col-begin}}
* [[History of intelligence testing]]
 
  +
{{col-2}}
* [[Intelligence (trait| Introduction to the trait of intelligence]]
 
* [[Intelligence tests]]
+
* [[Child prodigy]]
  +
* [[Cognitive assessment]]
* [[Triarchic theory of intelligence]]
 
  +
* [[Cultural Intelligence]]
* [[Theory of multiple intelligences]]
 
  +
* [[Curiosity quotient]]
* [[General intelligence factor]]
 
* [[Neuroscience and intelligence]]
 
* [[Fluid and crystallized intelligence]]
 
* [[Nature versus nurture]]
 
 
* [[Emotional intelligence]]
 
* [[Emotional intelligence]]
* [[Gifted]]
+
* [[EQ SQ theory]]
  +
* [[Genetics of intelligence]]
* [[SAT]]
 
* [[List of countries by IQ]]
+
* [[Graduate Record Examination]]
* [[IQ Societies]]
+
* [[Intellectual giftedness]]
  +
* [[IQ classification]]
===External links===
 
  +
* [[IQ and Global Inequality]]
  +
{{col-2}}
  +
* [[Late bloomer]]
  +
* [[Mental age]]
  +
* [[Nature versus nurture]]
  +
* [[Race Differences in Intelligence]]
  +
* [[Sentience Quotient]]
  +
* [[Social IQ]]
  +
* [[Spiritual intelligence]]
  +
* [[Theory of multiple intelligences]]
  +
* [[Triarchic theory of intelligence]]
  +
* [[IQ and the wealth of nations]]
  +
{{col-end}}
   
  +
==Notes==
* [http://www.mensa-test.com www.mensa-test.com] &mdash; Nice intelligence test (not a standard IQ test)
 
  +
{{reflist|2}}
* [http://www.iqte.st/onlineiqtestiqtest/index.html Online IQ testing IQ test]
 
* [http://www.sigmasociety.com/sigma_teste/sigma_sigma_teste.asp Sigma Test] &mdash; Available in 14 languages, accepted for admission in dozens of high IQ societies (with articles on Psychometry, history of IQ tests etc.)
 
* [http://www.soft32.com/download_125585.html IQ test trainer] &mdash; software for improving IQ and IQ score
 
* [http://members.shaw.ca/delajara/ IQ Comparison Site]
 
* [[American Psychological Association|APA]] &mdash; [http://www.lrainc.com/swtaboo/taboos/apa_01.html Intelligence: Knowns and Unknowns]
 
* [http://www.psych.utoronto.ca/~reingold/courses/intelligence/cache/1198yam.html Scientific American: Intelligence Considered]
 
* [http://www.psych.utoronto.ca/~reingold/courses/intelligence/cache/1198gottfred.html Scientific American: The General Intelligence Factor]
 
* [http://www.iqte.st iqte.st: Online IQ test reviews]
 
* [http://www.apa.org/science/testing_on_the_internet.pdf APA Committee on Online Psychological Tests and Assessment report]
 
* [http://hem.bredband.net/b153434/Index.htm Estimated IQs of the greatest geniuses]
 
* [http://www.volkmar-weiss.de/table.html Number of highly gifted relatives of high IQ people]
 
* [http://www.volkmar-weiss.de/lehrl.html The Basic Period of Individual Mental Speed, Underlying IQ]
 
* [http://www.eskimo.com/~miyaguch/hoeflin.html Uncommonly difficult IQ tests]
 
   
===References===
+
==References==
  +
*Carroll, J.B (1993). Human cognitive abilities: A survey of factor-analytical studies. New York: Cambridge University Press.
 
  +
*Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. ''Journal of Applied Psychology,'' 75:297–300.
*Carroll, J.B. (1993). Human cognitive abilities: A survey of factor-analytical studies. New York: Cambridge University Press.
 
  +
*{{cite journal |author=Duncan J, Burgess P, Emslie H |title=Fluid intelligence after frontal lobe lesions |journal=Neuropsychologia |volume=33 |issue=3 |pages=261–8 |year=1995 |pmid=7791994 |doi=10.1016/0028-3932(94)00124-8}}
*Coward, W.M. and Sackett, P.R. (1990). Linearity of ability-performance relationships: A reconfirmation. ''Journal of Applied Psychology,'' 75:297&#8211;300.
 
  +
*{{cite journal |author=Duncan J, Seitz RJ, Kolodny J, ''et al'' |title=A neural basis for general intelligence |journal=Science |volume=289 |issue=5478 |pages=457–60 |year=2000 |pmid=10903207 |doi=10.1126/science.289.5478.457}}
*Duncan, J., P. Burgess, and H. Emslie (1995) Fluid intelligence after frontal lobe lesions. Neuropsychologia, 33(3): p. 261-8.
 
  +
*Flynn, J.R. (1999). Searching for Justice: The discovery of IQ gains over time. American Psychologist, v. 54, p. 5-20
*Duncan, J., et al., A neural basis for general intelligence. Science, 2000. 289(5478): p. 457-60.
 
* Frey, M.C. and Detterman, D.K. (2003) Scholastic Assessment or ''g''? The Relationship Between the Scholastic Assessment Test and General Cognitive Ability. ''Psychological Science,'' 15(6):373&ndash;378. [http://www.gnxp.com/MT2/archives/2003_frey_and_detterman_IQ_SAT.pdf PDF]
+
*Frey, M.C. and Detterman, D.K. (2003) Scholastic Assessment or ''g''? The Relationship Between the Scholastic Assessment Test and General Cognitive Ability. ''Psychological Science,'' 15(6):373&ndash;378. [http://www.missouri.edu/~aab2b3/Detterman.g.Psychological.Science.pdf PDF]
  +
*{{cite journal |author=Gale CR, Deary IJ, Schoon I, Batty GD |title=IQ in childhood and vegetarianism in adulthood: 1970 British cohort study |journal=BMJ |volume=334 |issue=7587 |pages=245 |year=2007 |pmid=17175567 |doi=10.1136/bmj.39030.675069.55}}
 
*Gottfredson, L. S. (1997). "Why g matters: The complexity of everyday life." ''Intelligence'', 24(1), 79&ndash;132. [http://www.udel.edu/educ/gottfredson/reprints/1997whygmatters.pdf PDF]
 
*Gottfredson, L. S. (1997). "Why g matters: The complexity of everyday life." ''Intelligence'', 24(1), 79&ndash;132. [http://www.udel.edu/educ/gottfredson/reprints/1997whygmatters.pdf PDF]
*Gottfredson, L.S. (1998). The general intelligence factor. ''Scientific American Presents,'' 9(4):24&ndash;29. [http://www.udel.edu/educ/gottfredson/reprints/1998generalintelligencefactor.pdf PDF]
+
*Gottfredson, L.S. (1998). The general intelligence factor. ''Scientific American Presents,'' 9(4):24&ndash;29. [http://www.udel.edu/educ/gottfredson/reprints/1998generalintelligencefactor.pdf PDF]
 
*Gottfredson, L. S. (2005). Suppressing intelligence research: Hurting those we intend to help. In R. H. Wright & N. A. Cummings (Eds.), Destructive trends in mental health: The well-intentioned path to harm (pp. 155&ndash;186). New York: Taylor and Francis. [http://www.udel.edu/educ/gottfredson/reprints/2003suppressingintelligence.pdf Pre-print PDF] [http://www.udel.edu/educ/gottfredson/reprints/2005suppressingintelligence.pdf PDF]
 
*Gottfredson, L. S. (2005). Suppressing intelligence research: Hurting those we intend to help. In R. H. Wright & N. A. Cummings (Eds.), Destructive trends in mental health: The well-intentioned path to harm (pp. 155&ndash;186). New York: Taylor and Francis. [http://www.udel.edu/educ/gottfredson/reprints/2003suppressingintelligence.pdf Pre-print PDF] [http://www.udel.edu/educ/gottfredson/reprints/2005suppressingintelligence.pdf PDF]
* Gottfredson, L. S. (in press). "Social consequences of group differences in cognitive ability (Consequencias sociais das diferencas de grupo em habilidade cognitiva)". In C. E. Flores-Mendoza & R. Colom (Eds.), ''Introducau a psicologia das diferncas individuais''. Porto Allegre, Brazil: ArtMed Publishers. [http://www.udel.edu/educ/gottfredson/reprints/2004socialconsequences.pdf PDF]
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*Gottfredson, L. S. (in press). "Social consequences of group differences in cognitive ability (Consequencias sociais das diferencas de grupo em habilidade cognitiva)". In C. E. Flores-Mendoza & R. Colom (Eds.), ''Introdução à psicologia das diferenças individuais''. Porto Alegre, Brazil: ArtMed Publishers. [http://www.udel.edu/educ/gottfredson/reprints/2004socialconsequences.pdf PDF]
*Gray, J.R., C.F. Chabris, and T.S. Braver, Neural mechanisms of general fluid intelligence. Nat Neurosci, 2003. 6(3): p. 316-22.
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*{{cite journal |author=Gray JR, Chabris CF, Braver TS |title=Neural mechanisms of general fluid intelligence |journal=Nat. Neurosci. |volume=6 |issue=3 |pages=316–22 |year=2003 |pmid=12592404 |doi=10.1038/nn1014}}
*Gray, J.R. and P.M. Thompson, Neurobiology of intelligence: science and ethics. Nat Rev Neurosci, 2004. 5(6): p. 471-82.
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*{{cite journal |author=Gray JR, Thompson PM |title=Neurobiology of intelligence: science and ethics |journal=Nat. Rev. Neurosci. |volume=5 |issue=6 |pages=471–82 |year=2004 |pmid=15152197 |doi=10.1038/nrn1405}}
*{{cite journal | author=Haier RJ, Jung RE, Yeo RA, et al. | title=The neuroanatomy of general intelligence: sex matters | journal=NeuroImage | year=2005 | volume=25 | pages=320–327}}
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*{{cite journal |author=Haier RJ, Jung RE, Yeo RA, Head K, Alkire MT |title=The neuroanatomy of general intelligence: sex matters |journal=NeuroImage |volume=25 |issue=1 |pages=320–7 |year=2005 |pmid=15734366 |doi=10.1016/j.neuroimage.2004.11.019}}
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*Harris, J. R. (1998). The nurture assumption : why children turn out the way they do. New York, Free Press.
 
*Hunt, E. (2001). Multiple views of multiple intelligence. [Review of Intelligence Reframed: Multiple Intelligences for the 21st Century.]
 
*Hunt, E. (2001). Multiple views of multiple intelligence. [Review of Intelligence Reframed: Multiple Intelligences for the 21st Century.]
*Jensen, A.R. (1998). ''The ''g'' Factor.'' Praeger, Connecticut, USA.
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*Jensen, A.R. (1979). Bias in mental testing. New York: Free Press.
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*Jensen, A.R. (1998). ''The ''g'' Factor.'' Praeger, Connecticut, USA.
*Jensen, A.R. (2006). "Clocking the Mind: Mental Chronometry and Individual Differences." Elsevier Science. --->Highly anticipated new release scheduled for early June, 2006.
 
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*Jensen, A.R. (2006). "Clocking the Mind: Mental Chronometry and Individual Differences." Elsevier Science. --->New release scheduled for early June, 2006.
*McClearn, G. E., Johansson, B., Berg, S., Pedersen, N. L., Ahern, F., Petrill, S. A., & Plomin, R. (1997). Substantial genetic influence on cognitive abilities in twins 80 or more years old. Science, 276, 1560&ndash;1563.
 
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*{{cite journal |author=Klingberg T, Forssberg H, Westerberg H |title=Training of working memory in children with ADHD |journal=J Clin Exp Neuropsychol |volume=24 |issue=6 |pages=781–91 |year=2002 |pmid=12424652 |doi=}}
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*{{cite journal |author=McClearn GE, Johansson B, Berg S, ''et al'' |title=Substantial genetic influence on cognitive abilities in twins 80 or more years old |journal=Science |volume=276 |issue=5318 |pages=1560–3 |year=1997 |pmid=9171059 |doi=10.1126/science.276.5318.1560}}
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*Mingroni, M.A. (2004). "The secular rise in IQ: Giving heterosis a closer look". Intelligence 32: 65–83.
 
*Murray, Charles (1998). Income Inequality and IQ, AEI Press [http://www.aei.org/docLib/20040302_book443.pdf PDF]
 
*Murray, Charles (1998). Income Inequality and IQ, AEI Press [http://www.aei.org/docLib/20040302_book443.pdf PDF]
*Noguera, P.A. (2001). Racial politics and the elusive quest for excellence and equity in education. [http://www.inmotionmagazine.com/er/pnrp1.html In Motion Magazine article]
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*Noguera, P.A. (2001). Racial politics and the elusive quest for excellence and equity in education. [http://www.inmotionmagazine.com/er/pnrp1.html In Motion Magazine article]
 
*Plomin, R., DeFries, J. C., Craig, I. W., & McGuffin, P. (2003). ''Behavioral genetics in the postgenomic era''. Washington, DC: American Psychological Association.
 
*Plomin, R., DeFries, J. C., Craig, I. W., & McGuffin, P. (2003). ''Behavioral genetics in the postgenomic era''. Washington, DC: American Psychological Association.
 
*Plomin, R., DeFries, J. C., McClearn, G. E., & McGuffin, P. (2001). ''Behavioral genetics (4th ed.)''. New York: Worth Publishers.
 
*Plomin, R., DeFries, J. C., McClearn, G. E., & McGuffin, P. (2001). ''Behavioral genetics (4th ed.)''. New York: Worth Publishers.
* Rowe, D. C., W. J. Vesterdal, and J. L. Rodgers, "The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations," University of Arizona, 1997
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*Rowe, D. C., W. J. Vesterdal, and J. L. Rodgers, "The Bell Curve Revisited: How Genes and Shared Environment Mediate IQ-SES Associations," University of Arizona, 1997
*Schoenemann, P.T., M.J. Sheehan, and L.D. Glotzer, Prefrontal white matter volume is disproportionately larger in humans than in other primates. Nat Neurosci, 2005.
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*{{cite journal |author=Schoenemann PT, Sheehan MJ, Glotzer LD |title=Prefrontal white matter volume is disproportionately larger in humans than in other primates |journal=Nat. Neurosci. |volume=8 |issue=2 |pages=242–52 |year=2005 |pmid=15665874 |doi=10.1038/nn1394}}
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*{{cite journal |author=Shaw P, Greenstein D, Lerch J, ''et al'' |title=Intellectual ability and cortical development in children and adolescents |journal=Nature |volume=440 |issue=7084 |pages=676–9 |year=2006 |pmid=16572172 |doi=10.1038/nature04513}}
* Tambs K, Sundet JM, Magnus P, Berg K. "Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: a study of twins." Behav Genet. 1989 Mar;19(2):209&ndash;22. PMID 2719624.
 
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*{{cite journal |author=Tambs K, Sundet JM, Magnus P, Berg K |title=Genetic and environmental contributions to the covariance between occupational status, educational attainment, and IQ: a study of twins |journal=Behav. Genet. |volume=19 |issue=2 |pages=209–22 |year=1989 |pmid=2719624 |doi=}}
*Thompson, P.M., Cannon, T.D., Narr, K.L., Van Erp, T., Poutanen, V.-P., Huttunen, M., Lönnqvist, J., Standertskjöld-Nordenstam, C.-G., Kaprio, J., Khaledy, M., Dail, R., Zoumalan, C.I., Toga, A.W. (2001). "Genetic influences on brain structure." Nature Neuroscience 4, 1253-1258.
 
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*{{cite journal |author=Thompson PM, Cannon TD, Narr KL, ''et al'' |title=Genetic influences on brain structure |journal=Nat. Neurosci. |volume=4 |issue=12 |pages=1253–8 |year=2001 |pmid=11694885 |doi=10.1038/nn758}}
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*Wechsler, D. (1997). Wechsler Adult Intelligence Scale-Third Edition. San Antonio, TX: The Psychological Corporation.
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*Wechsler, D. (2003). Wechsler Intelligence Scale for Children-Fourth Edition. San Antonio, TX: The Psychological Corporation.
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==External links==
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===Collective statements===
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*[http://www.psychpage.com/learning/library/intell/mainstream.html The Wall Street Journal: Mainstream Science on Intelligence]
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*[http://www.udel.edu/educ/gottfredson/reprints/1997mainstream.pdf PDF Reprint - Mainstream science on intelligence: An editorial with 52 signatories, history, and bibliography.]
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*[http://www.psych.utoronto.ca/~reingold/courses/intelligence/cache/1198yam.html Scientific American: Intelligence Considered]
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Latest revision as of 14:40, 10 October 2016

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Social Processes: Methodology · Types of test


An intelligence quotient or IQ is a score derived from one of several different intelligence measures, standardized tests designed to measure intelligence. The term "IQ," a translation of the German Intelligenz-Quotient, was coined by the German psychologist William Stern in 1912 as a proposed method of scoring early modern children's intelligence tests such as those developed by Alfred Binet and Theodore Simon in the early 20th Century.[1] Although the term "IQ" is still in common use, the scoring of modern IQ tests such as the Wechsler Adult Intelligence Scale is now based on a projection of the subject's measured rank on the Gaussian bell curve with a center value (average IQ) of 100, and a standard deviation of 15 (different tests have various standard deviations, the Stanford-Binet IQ test has a standard deviation of 16).

IQ scores have been shown to correlate with such factors as morbidity and mortality,[2] parental social status,[3] and to a substantial degree, parental IQ: while IQ heritability has been investigated for nearly a century, controversy remains as to how much is heritable, and the mechanisms for heritability are still a matter of some debate.[4][5]

IQ scores are used in many contexts: as predictors of educational achievement or special needs, by social scientists who study the distribution of IQ scores in populations and the relationships between IQ score and other variables, and as predictors of job performance and income.[6][7][8][9][10][11]

The average IQ scores for many populations were rising at an average rate of three points per decade during the 20th century with most of the increase in the lower half of the IQ range: a phenomenon called the Flynn effect. It is disputed whether these changes in scores reflect real changes in intellectual abilities, or merely methodological problems with past testing.

History

In 1905 the French psychologist Alfred Binet published the first modern intelligence test, the Binet-Simon intelligence scale. His principal goal was to identify students who needed special help in coping with the school curriculum. Along with his collaborator Theodore Simon, Binet published revisions of his intelligence scale in 1908 and 1911, the last appearing just before his untimely death.

In 1912, the German psychologist William Stern coined the abbreviation "I.Q.", a translation of the German Intelligenz-Quotient ("intelligence quotient"), proposing that an individual's intelligence level be measured as a quotient of their estimated "mental age" and their chronological age. A further refinement of the Binet-Simon scale was published in 1916 by Lewis M. Terman, from Stanford University, who incorporated Stern's proposal, and this Stanford-Binet Intelligence Scale formed the basis for one of the modern intelligence tests that remains in common use.

Originally, IQ was calculated as a ratio with the formula

A 10-year-old who scored as high as the average 13-year-old, for example, would have an IQ of 130 (100*13/10).

In 1939 David Wechsler published the first intelligence test explicitly designed for an adult population, the Wechsler Adult Intelligence Scale, or WAIS. Subsequent to the publication of the WAIS, Wechsler extended his scale for younger ages, creating the Wechsler Intelligence Scale for Children, or WISC. The Wechsler scales contained separate subscores for verbal and performance IQ, thus being less dependent on overall verbal ability than early versions of the Stanford-Binet scale, and was the first intelligence scale to base scores on a standardized normal distribution rather than an age-based quotient: since age-based quotients worked only for children, this method was replaced by a projection of the measured rank on the Gaussian bell curve using an average IQ of 100 as the center value and a standard deviation of 15 or occasionally 16 or 24 points.

Thus, the modern IQ score is a mathematical transformation of a raw score on an IQ test, based on the rank of that score in a normalization sample,[12] Modern scores are sometimes referred to as "deviance IQ", while older method age-specific scores are referred to as "ratio IQ".

The two methodologies yield similar results near the middle of the bell curve, but the older ratio IQs yielded far higher scores for the intellectually gifted— for example, Marilyn vos Savant, who appeared in the Guinness Book of World Records, obtained a ratio IQ of 228. While this score could make sense using Binet's formula (and even then, only for a child), on the Gaussian curve model it would be an exceptional 7.9 standard deviations above the mean and hence virtually impossible in a population with a normal IQ distribution (see normal distribution). In addition, IQ tests like the Wechsler were not intended to reliably discriminate much beyond IQ 130, as they simply do not contain enough exceptionally difficult items.[2]

Since the publication of the WAIS, almost all intelligence scales have adopted the normal distribution method of scoring. The use of the normal distribution scoring method makes the term "intelligence quotient" an inaccurate description of the intelligence measurement, but "I.Q." still enjoys colloquial usage, and is used to describe all of the intelligence scales currently in use. The third edition of the WAIS (WAIS-III) is the most widely-used psychological test in the world, and the fourth edition of the WISC (WISC-IV) is the most widely used intelligence test for children.[How to reference and link to summary or text]

IQ testing

Structure

IQ tests come in many forms, and some tests use a single type of item or question, while others use several different subtests. Most tests yield both an overall score and individual subtest scores.

A typical IQ test requires the test subject to solve a fair number of problems in a set time under supervision. Most IQ tests include items from various domains, such as short-term memory, verbal knowledge, spatial visualization, and perceptual speed. Some tests have a total time limit, others have a time limit for each group of problems, and there are a few untimed, unsupervised tests, typically geared to measuring high intelligence. The most widely used standardized test for determining IQ is the WAIS (Wechsler Adult Intelligence Scale-Third Edition)[How to reference and link to summary or text]. The WAIS-III consists of fourteen subtests, seven verbal (Information, Comprehension, Arithmetic, Similarities, Vocabulary, Digit Span, and Letter-Number Sequencing) and seven performance (Digit Symbol-Coding, Picture Completion, Block Design, Matrix Reasoning, Picture Arrangement, Symbol Search, and Object Assembly).

Scoring

When standardizing an IQ test, a representative sample of the population is tested using each test question. IQ tests are calibrated in such a way as to yield a normal distribution, or "bell curve". Each IQ test, however, is designed and valid only for a certain IQ range. Because so few people score in the extreme ranges, IQ tests usually cannot accurately measure very low and very high IQs.

Various IQ tests measure a standard deviation with a different number of points. Thus, when an IQ score is stated, the standard deviation used should also be stated.

When an individual has scores that do not correlate with each other, there is a good reason to suspect a learning disability or other cause for this lack of correlation. Tests have been chosen for inclusion because they display the ability to use this method to predict later difficulties in learning.

An individual's IQ score may or may not be stable over the course of the individual's lifetime.[13]

IQ and general intelligence factor

Main article: General intelligence factor

Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking), with the summary score calculated from subtest scores. The average score, according to the bell curve, is 100. Individual subtest scores tend to correlate with one another, even when seemingly disparate in content.

Mathematical analysis of individuals' scores on the subtests of a single IQ test or the scores from a variety of different IQ tests (e.g., Stanford-Binet, WISC-R, Raven's Progressive Matrices, Cattell Culture Fair III, Universal Nonverbal Intelligence Test, Primary Test of Nonverbal Intelligence, and others) find that they can be described mathematically as measuring a single common factor and various factors that are specific to each test. This kind of factor analysis has led to the theory that underlying these disparate cognitive tasks is a single factor, termed the general intelligence factor (or g), that corresponds with the common-sense concept of intelligence.[14] In the normal population, g and IQ are roughly 90% correlated and are often used interchangeably.

Tests differ in their g-loading, which is the degree to which the test score reflects g rather than a specific skill or "group factor" (such as verbal ability, spatial visualization, or mathematical reasoning). g-loading and validity have been observed to be related in the sense that most IQ tests derive their validity mostly or entirely from the degree to which they measure g.[How to reference and link to summary or text]

Mental handicaps

Main article: Mental retardation

Individuals with an unusually low IQ score, varying from about 70 ("Educable Mentally Retarded") to as low as 20 (usually caused by a neurological condition), are considered to have developmental difficulties. However, there is no true IQ-based classification for developmental disabilities. But newer studies have proved that even though the perception of mental retardation in current social settings is of or tests as lower intelligence, some patients do show advanced abilities in terms of emotions, spatial abilities,cognitive and memory. Aside from current Savants and patients with Aspergers many children with Down Syndrome show a more introverted state of emotion but yet show uncanny ability between ethical and moral dilemmas.

Heritability

Main article: Inheritance of intelligence

The role of genes and environment (nature and nurture) in determining IQ is reviewed in Plomin et al. (2001, 2003).[15]. Until recently heritability was mostly studied in children. Various studies find the heritability of IQ between 0.4 and 0.8 in the United States;[16][13][17] that is, depending on the study, a little less than half to substantially more than half of the variation in IQ among the children studied was due to variation in their genes. The remainder was thus due to environmental variation and measurement error. A heritability in the range of 0.4 to 0.8 implies that IQ is "substantially" heritable.

The effect of restriction of range on IQ was examined by Matt McGue and colleagues, who write that "restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations ... IQ".[18] On the other hand, a 2003 study by Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman demonstrated that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes was close to zero.[19]

It is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. Surprisingly, the opposite occurs. Heritability measures in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood.[15] The American Psychological Association's 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the white population the heritability of IQ is "around .75". The Minnesota Study of Twins Reared Apart, a multiyear study of 100 sets of reared-apart twins which was started in 1979, concluded that about 70% of the variance in IQ was found to be associated with genetic variation. Some of the correlation of IQs of twins may be a result of the effect of the maternal environment before birth, shedding some light on why IQ correlation between twins reared apart is so robust.[20]

There are a number of points to consider when interpreting heritability:

  • A high heritability does not mean that the environment has no effect on the development of a trait, or that learning is not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions.[13]
  • A common error is to assume that because something is heritable it is necessarily unchangeable. This is wrong. Heritability does not imply immutability. As previously noted, heritable traits can depend on learning, and they may be subject to other environmental effects as well. The value of heritability can change if the distribution of environments (or genes) in the population is substantially altered. For example, an impoverished or suppressive environment could fail to support the development of a trait, and hence restrict individual variation. Differences in variation of heritability are found between developed and developing nations. This could affect estimates of heritability.[13] Another example is Phenylketonuria which previously caused mental retardation for everyone who had this genetic disorder. Today, this can be prevented by following a modified diet.
  • On the other hand, there can be effective environmental changes that do not change heritability at all. If the environment relevant to a given trait improves in a way that affects all members of the population equally, the mean value of the trait will rise without any change in its heritability (because the differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.[13]
  • Even in developed nations, high heritability of a trait within a given group has no necessary implications for the source of a difference between groups.[21][13]

Environment

Environmental factors play a role in determining IQ. Proper childhood nutrition appears critical for cognitive development; malnutrition can lower IQ. Other research indicates environmental factors such as prenatal exposure to toxins, duration of breastfeeding, and micronutrient deficiency can affect IQ.

It is well known that it is possible to increase one's IQ score by training, for example by regularly playing puzzle games, or strategy games like Chess. Musical training in childhood also increases IQ.[22] Recent studies have shown that training in using one's working memory may increase IQ.[23]

Family environment

In the developed world, nearly all personality traits show that, contrary to expectations, environmental effects actually cause non-related children raised in the same family ("adoptive siblings") to be as different as children raised in different families ([24]; [15]). There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, such that adoptive siblings are not more similar in IQ than strangers.[25] For IQ, adoption studies show that, after adolescence, adoptive siblings are no more similar in IQ than strangers (IQ correlation near zero), while full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.86), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (~0.0).[15] The American Psychological Association's report Intelligence: Knowns and Unknowns (1995)[13] states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. Do differences between children's family environments (within the normal range) produce differences in their intelligence test performance? The problem here is to disentangle causation from correlation. There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests. It also stated "We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence."[13]

A study of French children adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. The children came from poor backgrounds with IQs that initially averaged 77, putting them near retardation. Nine years later after adoption, they retook the I.Q. tests, and all of them did better. The amount they improved was directly related to the adopting family’s status. "Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."[26] On the other hand, the degree to which these increases persisted into adulthood are not clear from the study.

Stoolmiller (1999)[27] found that the range restriction of family environments that goes with adoption, that adopting families tend to be more similar on for example socio-economic status than the general population, means that role of the shared family environment have been underestimated in previous studies. Corrections for range correction applied to adoption studies indicate that socio-economic status could account for as much as 50% of the variance in IQ.[27] However, the effect of restriction of range on IQ for adoption studies was examined by Matt McGue and colleagues, who wrote that "restriction in range in parent disinhibitory psychopathology and family socio-economic status had no effect on adoptive-sibling correlations [in] IQ".[28]

Eric Turkheimer and colleagues (2003),[29] not using an adoption study, included impoverished US families. Results demonstrated that the proportions of IQ variance attributable to genes and environment vary nonlinearly with socio-economic status. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.[30] They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.[31]

Maternal (fetal) environment

A meta-analysis, by Devlin and colleagues in Nature (1997),[32] of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (fetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%. They argue that the shared maternal environment may explain the striking correlation between the IQs of twins, especially those of adult twins that were reared apart [How to reference and link to summary or text].

Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different than previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports.[33] They write that:

Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al [How to reference and link to summary or text] conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950),[How to reference and link to summary or text] in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion [How to reference and link to summary or text] . Research subsequent to the 1978 review largely reinforces Price’s hypothesis [How to reference and link to summary or text]

The Dickens and Flynn model

Dickens and Flynn (2001) [34] postulate that the arguments regarding the disappearance of the shared family environment should apply equally well to groups separated in time. This is contradicted by the Flynn effect. Changes here have happened too quickly to be explained by genetic heritable adaptation. This paradox can be explained by observing that the measure "heritability" includes both a direct effect of the genotype on IQ and also indirect effects where the genotype changes the environment, in turn affecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.[34][35]

IQ and the brain

Main article: Neuroscience and intelligence

In 2004, Richard Haier, professor of psychology in the Department of Pediatrics and colleagues at University of California, Irvine and the University of New Mexico used MRI to obtain structural images of the brain in 47 normal adults who also took standard IQ tests. The study demonstrated that general human intelligence appears to be based on the volume and location of gray matter tissue in the brain. Regional distribution of gray matter in humans is highly heritable. The study also demonstrated that, of the brain's gray matter, only about 6 percent appeared to be related to IQ.[36]

Many different sources of information have converged on the view that the frontal lobes are critical for fluid intelligence. Patients with damage to the frontal lobe are impaired on fluid intelligence tests (Duncan et al 1995). The volume of frontal grey (Thompson et al 2001) and white matter (Schoenemann et al 2005) have also been associated with general intelligence. In addition, recent neuroimaging studies have limited this association to the lateral prefrontal cortex. Duncan and colleagues (2000) showed using Positron Emission Tomography that problem-solving tasks that correlated more highly with IQ also activate the lateral prefrontal cortex. More recently, Gray and colleagues (2003) used functional magnetic resonance imaging (fMRI) to show that those individuals that were more adept at resisting distraction on a demanding working memory task had both a higher IQ and increased prefrontal activity. For an extensive review of this topic, see Gray and Thompson (2004).[37]

A study involving 307 children (age between six to nineteen) measuring the size of brain structures using magnetic resonance imaging (MRI) and measuring verbal and non-verbal abilities has been conducted (Shaw et al 2006). The study has indicated that there is a relationship between IQ and the structure of the cortex—the characteristic change being the group with the superior IQ scores starts with thinner cortex in the early age then becomes thicker than average by the late teens.[38]

Significant injuries isolated to one side of the brain, even those occurring at a young age, may not significantly affect IQ.[39]

Studies reach conflicting conclusions regarding the controversial idea that brain size correlates positively with IQ. Jensen and Reed (1993) claim no direct correlation exists in nonpathological subjects.[40] A more recent meta-analysis suggests otherwise.[41]

An alternative approach has sought to link differences in neural plasticity with intelligence (Garlick, 2002 [42]), and this view has recently received some empirical support (Shaw et al., 2006 [43]).

The Flynn effect

Main article: Flynn effect

The Flynn effect (aka the "Lynn-Flynn effect") was discovered by Richard Lynn in 1982, but is named after James R. Flynn, a New Zealand based political scientist. Flynn showed that IQ scores worldwide appear to be slowly rising at a rate of around three IQ points per decade [44]. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis [45]. Tests are therefore renormalized occasionally to obtain mean scores of 100, for example WISC-R (1974), WISC-III (1991) and WISC-IV (2003). Hence it is difficult to compare IQ scores measured years apart, unless this is compensated for.

The Flynn effect may have ended in some developed nations starting in the mid 1990s. Teasdale & Owen (2005) "report intelligence test results from over 500,000 young Danish men, tested between 1959 and 2004, showing that performance peaked in the late 1990s, and has since declined moderately to pre-1991 levels." They speculate that "a contributing factor in this recent fall could be a simultaneous decline in proportions of students entering 3-year advanced-level school programs for 16–18 year olds."[46]

In 2004, Jon Martin Sundet of the University of Oslo and colleagues published an article documenting scores on intelligence tests given to Norwegian conscripts between the 1950s and 2002, showing that the increase in scores of general intelligence stopped after the mid-1990s and in numerical reasoning subtests, declined.[47]

Group differences

Among the most controversial issues related to the study of intelligence is the observation that intelligence measures such as IQ scores vary between populations. While there is little scholarly debate about the existence of some of these differences, the reasons remain highly controversial both within academia and in the public sphere.

Health and IQ

Main article: Health and intelligence

Persons with a higher IQ have generally lower adult morbidity and mortality. Post-Traumatic Stress Disorder,[48] severe depression, [49][50] and schizophrenia are less prevalent in higher IQ bands. High IQ has also been positively correlated with a lower risk for heart disease[citation needed].

A study of 11,282 individuals in Scotland who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, found an "inverse linear association" between childhood IQ scores and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained even after accounting for factors such as the child's socioeconomic background.[51] Research in Scotland has also shown that a 15-point lower IQ meant people had a fifth less chance of seeing their 76th birthday, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.[52]

A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer's Disease and other forms of dementia. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia.[53] However, when diagnosing individuals with a higher level of cognitive ability, in this study those with IQ's of 120 or more,[54] patients should not be diagnosed from the standard norm but from an adjusted high-IQ norm that measured changes against the individual's higher ability level. In 2000, Whalley and colleagues published a paper in the journal Neurology, which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.[55]

Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood-brain barrier is less effective. Such impairment may sometimes be permanent, or may sometimes be partially or wholly compensated for by later growth. Several harmful factors may also combine, possibly causing greater impairment.

Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.[56]

In terms of the effect of one's intelligence on health, high childhood IQ correlates with one's chance of becoming a vegetarian in adulthood (Gale, CR. IQ in childhood and vegetarianism in adulthood: 1970 British cohort study. British Journal of Medicine 334 (7587): 245.), and inversely correlates with the chances of smoking (Taylor, MD. Childhood IQ and social factors on smoking behaviour, lung function and smoking-related outcomes in adulthood: linking the Scottish Mental Survey 1932 and the Midspan studies. British Journal of Health Psychology 10 (3): 399-401.), becoming obese, and having serious traumatic accidents in adulthood.

Gender and IQ

Main article: Sex and intelligence

Most studies claim that despite sometimes significant differences in subtest scores, men and women have quite similar average IQ[How to reference and link to summary or text]. Some studies claim that men outperform women on average by 3-4 IQ points.[57][58] However, during a revision of some tests in the 1940's women outperformed men, which urged test makers to change the tests until men were able to score equally.[59] Some studies claim that women perform better on tests of memory and verbal proficiency, while men perform better on tests of mathematical and spatial ability[How to reference and link to summary or text]. Male scores display a higher variance: there are more men than women identified with both very high and very low IQs[How to reference and link to summary or text].

Race and IQ

Main article: Race and intelligence

Much research has been devoted to the extent and potential causes of racial group differences in IQ.

Positive correlations with IQ

While IQ is sometimes treated as an end unto itself, scholarly work on IQ focuses to a large extent on IQ's validity, that is, the degree to which IQ correlates with outcomes such as job performance, social pathologies, or academic achievement. Different IQ tests differ in their validity for various outcomes. Traditionally, correlation for IQ and outcomes is viewed as a means to also predict performance; however, because IQ is a known social artifact, readers should distinguish between prediction in the hard sciences and the social sciences.

Validity is the correlation between score (in this case cognitive ability, as measured, typically, by a paper-and-pencil test) and outcome (in this case job performance, as measured by a range of factors including supervisor ratings, promotions, training success, and tenure), and ranges between −1.0 (the score is perfectly wrong in predicting outcome) and 1.0 (the score perfectly predicts the outcome). See validity (psychometric).

Research shows that general intelligence plays an important role in many valued life outcomes. In addition to academic success, IQ correlates to some degree with job performance (see below), socioeconomic advancement (e.g., level of education, occupation, and income), and "social pathology" (e.g., adult criminality, poverty, unemployment, dependence on welfare, children outside of marriage). Recent work has demonstrated links between general intelligence and health, longevity, and functional literacy. Correlations between g and life outcomes are pervasive, though IQ does not correlate with subjective self-reports of happiness. IQ and g correlate highly with school performance and job performance, less so with occupational prestige, moderately with income, and to a small degree with law-abiding behaviour. IQ does not explain the inheritance of economic status and wealth.

Other tests

One study found a correlation of .82 between g and SAT scores.[3] Another correlation of .81 between g and GCSE scores.[4]

Correlations between IQ scores (general cognitive ability) and achievement test scores are reported to be .81 by Deary and colleagues, with the percentage of variance accounted for by general cognitive ability ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design"[60]

School performance

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995)[13] Wherever it has been studied, children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. The correlation between IQ scores and grades is about .50. However, this means that they explain only 25% of the variance. Successful school learning depends on many personal characteristics other than intelligence, such as memory, persistence, interest in school, and willingness to study.

Correlations between IQ scores and total years of education are about .55, implying that differences in psychometric intelligence account for about 30% of the outcome variance. Many occupations can only be entered through professional schools which base their admissions at least partly on test scores: the MCAT, the GMAT, the GRE, the DAT, the LSAT, etc. Individual scores on admission-related tests such as these are certainly correlated with scores on tests of intelligence. It is partly because intelligence test scores predict years of education that they also predict occupational status, and income to a smaller extent.

Job performance

According to Schmidt and Hunter, "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability."[61] The validity depends on the type of job and varies across different studies, ranging from 0.2 to 0.6 [62]. However IQ mostly correlates with cognitive ability only if IQ scores are below average and this rule has many (about 30 %) exceptions for people with average and higher IQ scores [63]. Also, IQ is related to the "academic tasks" (auditory and linguistic measures, memory tasks, academic achievement levels) and much less related to tasks where even precise hand work ("motor functions") are required [64]

A meta-analysis (Hunter and Hunter, 1984)[62] which pooled validity results across many studies encompassing thousands of workers (32,124 for cognitive ability), reports that the validity of cognitive ability for entry-level jobs is 0.54, larger than any other measure including job try-out (0.44), experience (0.18), interview (0.14), age (−0.01), education (0.10), and biographical inventory (0.37). This implies that, across a wide range of occupations, intelligence test performance accounts for some 29% of the variance in job performance.

According to Marley Watkins and colleagues, IQ is a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.[65] Treena Eileen Rohde and Lee Anne Thompson write that general cognitive ability but not specific ability scores predict academic achievement, with the exception that processing speed and spatial ability predict performance on the SAT math beyond the effect of general cognitive ability.[66]

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995)[13] states that other individual characteristics such as interpersonal skills, aspects of personality, etc., are probably of equal or greater importance, but at this point we do not have equally reliable instruments to measure them.[13]

Income

Some researchers claim that "in economic terms it appears that the IQ score measures something with decreasing marginal value. It is important to have enough of it, but having lots and lots does not buy you that much."[67][68]

Other studies show that ability and performance for jobs are linearly related, such that at all IQ levels, an increase in IQ translates into a concomitant increase in performance [69]. Charles Murray, coauthor of The Bell Curve, found that IQ has a substantial effect on income independently of family background [70].

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995)[13] states that IQ scores account for about one-fourth of the social status variance and one-sixth of the income variance. Statistical controls for parental SES eliminate about a quarter of this predictive power. Psychometric intelligence appears as only one of a great many factors that influence social outcomes.[13]

One reason why some studies claim that IQ only accounts for a sixth of the variation in income is because many studies are based on young adults (many of whom have not yet completed their education). On pg 568 of The g factor, Arthur Jensen claims that although the correlation between IQ and income averages a moderate 0.4 (one sixth or 16% of the variance), the relationship increases with age, and peaks at middle age when people have reached their maximum career potential. In the book, a Question of Intelligence, Daniel Seligman cites an IQ income correlation of 0.5 (25% of the variance).

A 2002 study[71] further examined the impact of non-IQ factors on income and concluded that an offspring's inherited wealth, race, and schooling are more important as factors in determining income than IQ.

Other correlations with IQ

In addition, IQ and its correlation to health, violent crime, gross state product, and government effectiveness are the subject of a 2006 paper in the publication Intelligence. The paper breaks down IQ averages by U.S. states using the federal government's National Assessment of Educational Progress math and reading test scores as a source.[72]

There is a correlation of -.19 between IQ scores and number of juvenile offences in a large Danish sample; with social class controlled, the correlation dropped to -. 17. Similarly, the correlations for most "negative outcome" variables are typically smaller than .20, which means that test scores are associated with less than 4% of their total variance. It is important to realize that the causal links between psychometric ability and social outcomes may be indirect. Children who are unsuccessful in - and hence alienated from - school may be more likely to engage in delinquent behaviours for that very reason, compared to other children who enjoy school and are doing well.[13]

IQ is also associated with certain diseases.

The book IQ and the Wealth of Nations claims to show that the GDP/person of a nation can in large part be explained by the average IQ score of its citizens. This claim has been both disputed and supported in peer-reviewed papers. The data used have also been questioned.

Tambs et al. (1989)[73] found that occupational status, educational attainment, and IQ are individually heritable; and further found that "genetic variance influencing educational attainment … contributed approximately one-fourth of the genetic variance for occupational status and nearly half the genetic variance for IQ". In a sample of U.S. siblings, Rowe et al. (1997)[74] report that the inequality in education and income was predominantly due to genes, with shared environmental factors playing a subordinate role.

Some argue that IQ scores are used as an excuse for not trying to reduce poverty or otherwise improve living standards for all. Claimed low intelligence has historically been used to justify the feudal system and unequal treatment of women (but note that many studies find identical average IQs among men and women; see sex and intelligence). In contrast, others claim that the refusal of "high-IQ elites" to take IQ seriously as a cause of inequality is itself immoral.[75]

Public policy

Main article: Intelligence and public policy

In the United States, certain public policies and laws regarding military service,[76] education, public benefits,[77] crime,[78]and employment incorporate an individual's IQ or similar measurements into their decisions. However, in 1971 the U.S. Supreme Court had banned the use of IQ tests in employment, except in very rare cases[79]. Internationally, certain public policies, such as improving nutrition and prohibiting neurotoxins, have as one of their goals raising or preventing a decline in intelligence.

Criticism and views

Binet

Alfred Binet did not believe that IQ test scales qualified to measure intelligence. He neither invented the term "intelligence quotient" nor supported its numerical expression. He stated:

The scale, properly speaking, does not permit the measure of intelligence, because intellectual qualities are not superposable, and therefore cannot be measured as linear surfaces are measured. (Binet 1905)

Binet had designed the Binet-Simon intelligence scale in order to identify students who needed special help in coping with the school curriculum. He argued that with proper remedial education programs, most students regardless of background could catch up and perform quite well in school. He did not believe that intelligence was a measurable fixed entity.

Binet cautioned:

Some recent thinkers seem to have given their moral support to these deplorable verdicts by affirming that an individual's intelligence is a fixed quantity, a quantity that cannot be increased. We must protest and react against this brutal pessimism; we must try to demonstrate that it is founded on nothing.[80]

The Mismeasure of Man

Some scientists dispute psychometrics entirely. In The Mismeasure of Man professor Stephen Jay Gould argued that intelligence tests were based on faulty assumptions and showed their history of being used as the basis for scientific racism. He wrote:

…the abstraction of intelligence as a single entity, its location within the brain, its quantification as one number for each individual, and the use of these numbers to rank people in a single series of worthiness, invariably to find that oppressed and disadvantaged groups—races, classes, or sexes—are innately inferior and deserve their status. (pp. 24–25)

He spent much of the book criticizing the concept of IQ, including a historical discussion of how the IQ tests were created and a technical discussion of why g is simply a mathematical artifact. Later editions of the book included criticism of The Bell Curve.

Gould does not dispute the stability of test scores, nor the fact that they predict certain forms of achievement. He does argue, however, that to base a concept of intelligence on these test scores alone is to ignore many important aspects of mental ability.

Relation between IQ and intelligence

See also: Intelligence

Several other ways of measuring intelligence have been proposed. Daniel Schacter, Daniel Gilbert, and others have moved beyond general intelligence and IQ as the sole means to describe intelligence.[81]

Test bias

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995)[13] states that that IQ tests as predictors of social achievement are not biased against people of African descent since they predict future performance, such as school achievement, similarly to the way they predict future performance for European descent.[13]

However, IQ tests may well be biased when used in other situations. A 2005 study finds some evidence that the WAIS-R is not culture-fair for Mexican Americans.[82] Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.[83][84] Standard intelligence tests, such as the Stanford-Binet, are often inappropriate for children with autism; the alternative of using developmental or adaptive skills measures are relatively poor measures of intelligence in autistic children, and have resulted in incorrect claims that a majority of children with autism are mentally retarded.[85]

Outdated methodology

A 2006 paper argues that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s."[86]It also claims that some of the most influential recent studies on group differences in intelligence, in order to show that the tests are unbiased, use outdated methodology.

The view of the American Psychological Association

In response to the controversy surrounding The Bell Curve, the American Psychological Association's Board of Scientific Affairs established a task force in 1995 to write a consensus statement on the state of intelligence research which could be used by all sides as a basis for discussion. The full text of the report is available through several websites.[13]

In this paper the representatives of the association regret that IQ-related works are frequently written with a view to their political consequences: "research findings were often assessed not so much on their merits or their scientific standing as on their supposed political implications".

The task force concluded that IQ scores do have high predictive validity for individual differences in school achievement. They confirm the predictive validity of IQ for adult occupational status, even when variables such as education and family background have been statistically controlled. They agree that individual (but specifically not population) differences in intelligence are substantially influenced by genetics.

They state there is little evidence to show that childhood diet influences intelligence except in cases of severe malnutrition. They agree that there are no significant differences between the average IQ scores of males and females. The task force agrees that large differences do exist between the average IQ scores of blacks and whites, and that these differences cannot be attributed to biases in test construction. The task force suggests that explanations based on social status and cultural differences are possible, and that environmental factors have raised mean test scores in many populations. Regarding genetic causes, they noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.

The APA journal that published the statement, American Psychologist, subsequently published eleven critical responses in January 1997, several of them arguing that the report failed to examine adequately the evidence for partly-genetic explanations.


High IQ societies

Main article: High IQ society

A high IQ society is an organization that limits membership to people who are within a certain high percentile of IQ test results. The most well-known is Mensa International, which requires members to score in the top 2% of a standardized IQ test.

Reference charts

Main article: IQ reference chart

IQ reference charts are tables suggested by psychologists to divide intelligence ranges in various categories.

See also

Notes

  1. i.e. as a quotient of "mental age" and "chronological age."
  2. Cervilla et al. Premorbid cognitive testing predicts the onset of dementia and Alzheimer's disease better than and independently of APOE genotype. Psychiatry 2004;75:1100-1106..
  3. Intelligence: Knowns and Unknowns (Report of a Task Force established by the Board of Scientific Affairs of the American Psychological Association - Released August 7, 1995 - A slightly edited version was published in the American Psychologist, Feb 1996. Official Journal of the APA)
  4. The heritability of IQ. Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):468-71.
  5. The same study suggests that the heritable component of IQ becomes more significant with age.
  6. Whalley et al.. Childhood mental ability and dementia. Neurology 2000;55:1455-1459..
  7. Naomi Breslau, PhD; Victoria C. Lucia, PhD; German F. Alvarado, MD, MPH. Intelligence and Other Predisposing Factors in Exposure to Trauma and Posttraumatic Stress Disorder. A Follow-up Study at Age 17 Years. Arch Gen Psychiatry. 2006;63:1238-1245.
  8. Effects of major depression on estimates of intelligence Sackeim HA, Freeman J, McElhiney M, Coleman E, Prudic J, Devanand DP. J Clin Exp Neuropsychol. 1992 Mar;14(2):268-88.
  9. Improvement of cognitive functioning in mood disorder patients with depressive symptomatic recovery during treatment: An exploratory analysis LAURA MANDELLI, Psy. D, ALESSANDRO SERRETTI, MD, CRISTINA COLOMBO, MD, MARCELLO FLORITA, Psy. D, ALESSIA SANTORO, Psy. D, DAVID ROSSINI, MD, RAFFAELLA ZANARDI, MD and ENRICO SMERALDI, MD. Psychiatry and Clinical Neurosciences Volume 60 Issue 5 Page 598 - October 2006
  10. Whalley and Deary. Longitudinal cohort study of childhood IQ and survival up to age 76. British Medical Journal 2001, 322:819-819.
  11. Debbie A. Lawlor, University of Bristol, Heather Clark, University of Aberdeen, David A. Leon, London School of Hygiene & Tropical Medicine. Associations Between Childhood Intelligence and Hospital Admissions for Unintentional Injuries in Adulthood: The Aberdeen Children of the 1950s Cohort Study. American Journal of Public Health, December 2006.
  12. see: quantile, percentile, percentile rank.
  13. 13.00 13.01 13.02 13.03 13.04 13.05 13.06 13.07 13.08 13.09 13.10 13.11 13.12 13.13 13.14 13.15 13.16 Neisser et al.. Intelligence: Knowns and Unknowns. Board of Scientific Affairs of the American Psychological Association.
  14. Linda S. Gottfredson. The General Intelligence Factor. Scientific American.
  15. 15.0 15.1 15.2 15.3 Plomin et al. (2001, 2003)
  16. R. Plomin, N. L. Pedersen, P. Lichtenstein and G. E. McClearn (05 1994). Variability and stability in cognitive abilities are largely genetic later in life. Behavior Genetics 24 (3).
  17. Thomas J. Bouchard Jr.; David T. Lykken; Matthew McGue; Nancy L. Segal; Auke Tellegen. Minnesota Study of Twins Reared Apart. National Institutes of Health / Science, Oct 12, 1990 v250 n4978 p223(6).
  18. http://dx.doi.org/10.1007/s10519-007-9142-7
  19. Socioeconomic status modifies heritability of iq in young children Psychological Science, Volume 14 Issue 6 Page 623-628, November 2003
  20. The heritability of IQ. Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):468-71.
  21. See: Ethnic Differences in Children's Intelligence Test Scores: Role of Economic Deprivation, Home Environment, and Maternal Characteristics
  22. Schellenberg, E. G. (2004). "Music lessons enhance IQ." Psychol Sci 15(8): 511-4.
  23. (Klingberg et al., 2002)
  24. Harris, 1998
  25. Genetic and environmental influences on adult intelligence and special mental abilities. Human Biology, 70, 257–279. 1998
  26. David L. Kirp. After the Bell Curve. New York Times Magazine.
  27. 27.0 27.1 Stoolmiller, M. (1999). Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior-genetic adoption studies. Psychological Bulletin, 125, 392-409.
  28. http://dx.doi.org/10.1007/s10519-007-9142-7
  29. Eric Turkheimer and colleagues (2003)
  30. Socioeconomic status modifies heritability of iq in young children Eric Turkheimer, Andreana Haley, Mary Waldron, Brian D'Onofrio, Irving I. Gottesman. Psychological Science 14 (6), 623–628. 2003
  31. New Thinking on Children, Poverty & IQ November 10, 2003 Connect for Kids
  32. The heritability of IQ. Devlin B, Daniels M, Roeder K. Nature. 1997 Jul 31;388(6641):417-8.
  33. DOI:10.1002/neu.10160
  34. 34.0 34.1 Dickens and Flynn (2001) Cite error: Invalid <ref> tag; name "DickensFlynn2001" defined multiple times with different content
  35. William T. Dickens and James R. Flynn, "The IQ Paradox: Still Resolved," Psychological Review 109, no. 4 (2002).
  36. Richard Haier. Human Intelligence Determined by Volume and Location of Gray Matter Tissue in Brain. Brain Research Institute, UC Irvine College of Medicine.
  37. Jeremy R. Gray, Psychology Department, Yale University, and Paul M. Thompson, Laboratory of Nero Imaging, Department of Neurology, University of California, Los Angeles School of Medicine. Neurobiology of Intelligence: Science and Ethics. Nature Publishing Group, Volume 5.
  38. Nicholas Wade. Scans Show Different Growth for Intelligent Brains. Brain Research Institute, UCLA...
  39. Bava S, Ballantyne AO, Trauner DA. Disparity of verbal and performance IQ following early bilateral brain damage. J Int Neuropsychol Soc. 2004 Jan;10(1):82-90. Pubmed link
  40. Reed, T.E., & Jensen, A.R. 1993. Cranial capacity: new Caucasian data and comments on Rushton's claimed Mongoloid-Caucasoid brain-size differences. Intelligence, 17, 423-431
  41. McDaniel, M.A. (2005) Big-brained people are smarter: A meta-analysis of the relationship between in vivo brain volume and intelligence. Intelligence, 33, 337-346. PDF
  42. Garlick, D. (2002). Understanding the nature of the general factor of intelligence: The role of individual differences in neural plasticity as an explanatory mechanism. Psychological Review, 109, 116-136. [1]
  43. Shaw, P., Greenstein, D., Lerch, J., Clasen, L., Lenroot, R., Gogtay, N., Evans, A., Rapoport, J., & Giedd, J. (2006). Intellectual ability and cortical development in children and adolescents. Nature, 440, 676-679.
  44. (Flynn, 1999)
  45. Mingroni, Michael A. (2007), Resolving the IQ Paradox: Heterosis as a Cause of the Flynn Effect and Other Trends, pp. 806-829 
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