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Demographic studies have indicated that in humans, fertility and intelligence tend to be negatively correlated, that is to say, the more intelligent, as measured by IQ, exhibit a lower total fertility rate than the less intelligent. Other correlates of fertility include income, diet, and educational attainment which are also correlated with intelligence.
The link between intelligence and fertility was of concern to eugenicists
Some of the first studies into the subject were carried out on individuals living before the advent of IQ testing, in the late 19th century, by looking at the fertility of men listed in WHO's WHO, these individuals being presumably of high intelligence[How to reference and link to summary or text]. These men, taken as a whole, had few children, implying a negative correlation.
Godfrey Thomson was active in this field, conducting some of the first UK representative sample research, which demonstrated that the relationship was negative.Cite error: Invalid
invalid names, e.g. too many But several reviewers considered the findings premature, arguing that the samples were nationally unrepresentative, generally being confined to whites born between 1910 and 1940 in the Great Lakes States. Other researchers began to report a negative correlation in the 1960s after two decades of neutral or positive fertility. 
In 1982, Daniel Vining sought to address these issues in a large study on the fertility of over 10,000 individuals throughout the United States, who were then aged 25 to 34. The average fertility in his study was correlated at -0.86 with IQ for white women and -0.96 for black women.  In considering these results along with those from earlier researchers, Vining wrote that "in periods of rising birth rates, persons with higher intelligence tend to have fertility equal to, if not exceeding, that of the population as a whole."
To address the concern that the fertility of this sample could not be considered complete, Vining carried out a follow-up study for the same sample 18 years later, reporting the same, though slightly decreased, negative correlation between IQ and fertility.
Regardless of the methodology employed, later research has generally supported that of Vining. In a 1988 study, Retherford and Sewell examined the association between the measured intelligence and fertility of over 9,000 high school graduates in Wisconsin in 1957, and confirmed the inverse relationship between IQ and fertility for both sexes, but much more so for females.
In a 1999 study Richard Lynn examined the relationship between the intelligence of adults aged 40 and above and their numbers of children and their siblings. Data were collected from the 1994 National Opinion Research Center survey among a representative sample of 2992 English-speaking individuals aged 18 years. Findings revealed that weak negative correlations of -0.05 and -0.09, respectively were found. Further analysis showed that the negative correlation was present only in females. The correlation for females between intelligence and ideal number of children was effectively zero.
Fertility and education
Another way of checking the negative relationship between IQ and fertility is to consider the relationship which educational attainment has to fertility, since education is known to be a reasonable proxy for IQ, correlating with IQ at .55; in a 1999 study examining the relationship between IQ and education in a large national sample, David Rowe and others found not only that achieved education had a high heritability (.68) and that half of the variance in education was explained by an underlying genetic component shared by IQ, education, and SES. One study investigating fertility and education[How to reference and link to summary or text] carried out in 1991 found that high school dropouts in America had the most children (2.5 on average), with high school graduates having fewer children, and college graduates having the fewest children (1.56 on average).
Birth control and intelligence
Among a sample of women using a reliable form of birth control, success rates were related to IQ, with the percentages of high, medium and low IQ women having unwanted births during a three-year interval being 3%, 8% and 11%, respectively.  Since the effectiveness of Birth Control is directly correlated with proper usage, an alternate interpretation of the data would indicate lower IQ women were more prone to misuse of Birth Control. Another study found that after an unwanted pregnancy has occurred, higher IQ couples are more likely to obtain abortions ; and unmarried teenage girls who become pregnant are found to be more likely to carry their babies to term if they are doing poorly in school. Conversely, while desired family size is apparently the same for women of all IQ levels, highly educated women are found to be more likely to say that they desire more children than they have, indicating a "deficit fertility" in the highly intelligent. In her review of reproductive trends in the United States, Van Court argues that "each factor - from initially employing some form of contraception, to successful implementation of the method, to termination of an accidental pregnancy when it occurs - involves selection against intelligence." 
Although much of the research into intelligence and fertility has been restricted to individuals within a single nation (most of them living within the United States), Steven Shatz has recently extended the research internationally; he finds that "There is a strong tendency for countries with lower national IQ scores to have higher fertility rates and for countries with higher national IQ scores to have lower fertility rates."
However, as nations with higher IQ scores have access to more resources, and thus, prophylactics and fertility education, than nations with lower IQ scores, the birth rate would be expected to be lower.
A theory to explain the fertility-intelligence relationship is that fertility is conversely driven by income. This well studied correlation is known as the demographic-economic paradox, which shows an inverse correlation between wealth and fertility within and between nations. The higher the degree of education and GDP per capita of a human population, subpopulation or social stratum, the fewer children are born. In a 1974 UN population conference in Bucharest, Karan Singh, a former minister of population in India, illustrated this trend by stating "Development is the best contraceptive"  Although, this again, would indicate a higher production of children, and not necessarily a higher capacity to produce children.
Other possible causes for this data would include effects ranging from availability and proliferation of prophylactics and neurological disorders which would create a social stigma associated with producing children, to race and accidents which would compromise ability to produce children. None of the listed would affect the genetic capability, but rather the social capability.
Natural arguments against Negative Correlation Between Fertility and Intelligence
There is a strong form of self-evident argumentation which argues that, even if Fertility were negatively correlated to intelligence, natural selection would still interfere with these traits in a way which ensures that high levels of intelligence are still selected for overall. However, competitive societies (whether they be socialist or capitalist) should preferentially select for individuals with higher levels of intelligence - especially given the ubiquitousness of war, competition for economic resources and the inevitable struggle for existence which is associated with all human existence.
- Demographic-economic paradox
- Total fertility rate
- List of countries and territories by fertility rate
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