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Intellectual disability
ICD-10 F70-F79
ICD-9 317-319
OMIM [1]
DiseasesDB 4509
MedlinePlus [2]
eMedicine med/3095 neuro/605
MeSH {{{MeshNumber}}}

Intellectual disability (ID) is a developmental disability. It involves significant limitations both in intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills.

People who acquired intellectual difficulties (e.g. through traumatic brain injury or lead poisoning) are diagnosed with neurocognitive disorder under the DSM-5.[1]


LR24 F Intellectual Disability Child

People with ID are sometimes slower.

Children with ID may learn to sit up, to crawl, or to walk later than other children, or they may learn to talk later. Both adults and children with ID may also exhibit the following characteristics:

The limitations of cognitive functioning will cause a child with ID to learn and develop more slowly than a typical child. Children may take longer to learn language, develop social skills, and take care of their personal needs such as dressing or eating. Learning will take them longer, require more repetition, and skills may need to be adapted to their learning level. Nevertheless, virtually every child is able to learn, develop and become participating members of the community.

In early childhood, borderline intellectual disability (IQ 71-84) and mild intellectual disability (IQ 60–70) may not be obvious, and may not be identified until children begin school. Even when poor academic performance is recognized, it may take expert assessment to distinguish mild ID from learning disability, developmental conditions like ADHD, and emotional conditions. As individuals with mild ID reach adulthood, many learn to live independently and maintain gainful employment.

Moderate intellectual disability (IQ 50–60) is nearly always apparent within the first years of life. Children with moderate ID will require considerable supports in school, at home, and in the community in order to participate fully. As adults they may live with their parents, in a supportive group home, or even semi-independently with significant supportive services to help them, for example, manage their finances.

A person with severe intellectual disability or profound intellectual disability will need lifelong intensive support and supervision.


The DSM-5 describes 3 criteria for intellectual disability:[1]

  1. An IQ below 70
  2. Significant limitations in two or more areas of adaptive behavior (as measured by an adaptive behavior rating scale, i.e. communication, social skills, personal independence at home or in community settings, and school/work functioning)
  3. Evidence that the limitations became apparent before the age of 18

It is formally diagnosed by professional assessment of intelligence and adaptive behavior.

IQ below 70[]

The first English-language IQ test, the Terman-Binet, was adapted from an instrument used to measure potential to achieve developed by Binet in France. Terman translated the test and employed it as a means to measure a person's intellectual capacity based on their oral language, vocabulary, numerical reasoning, memory, motor speed and analysis skills. The mean score on the currently available IQ tests is 100, with a standard deviation of 15 (WAIS/WISC-IV) or 16 (Stanford-Binet).

Sub-average intelligence is generally considered to be present when an individual scores two standard deviatons below the test mean. Factors other than cognitive ability (depression, anxiety, etc.) can contribute to low IQ scores. The evaluator should rule them out prior to concluding that measured IQ is "significantly below average".

The following ranges, based on Standard Scores of intelligence tests, reflect the categories of the American Association on Intellectual and Developmental Disabilities, the Diagnostic and Statistical Manual of Mental Disorders]-IV-TR, and the International Classification of Diseases-10:

Class IQ
Profound intellectual disability Below 20
Severe intellectual disability 20–34
Moderate intellectual disability 35–49
Mild mental intellectual disability 50–69
Borderline intellectual functioning 70–79

Since the diagnosis is not based only on IQ scores, but must also take into consideration the person's adaptive functioning, the diagnosis is not made rigidly. It encompasses intellectual scores, adaptive functioning scores from an adaptive behavior rating scale based on descriptions of known abilities provided by someone familiar with the person, and also the observations of the assessment examiner who is able to find out directly from the person what he or she can understand, communicate, and the like.

Significant limitations in two or more areas of adaptive behavior[]

Adaptive behavior, or adaptive functioning, refers to the skills needed to live independently (or at the minimally acceptable level for age). To assess adaptive behavior, professionals compare the functional abilities of a child to those of other children of similar age. To measure adaptive behavior, professionals use structured interviews, with which they systematically elicit information about the person's functioning in the community from someone who knows them well. There are many adaptive behavior scales, and accurate assessment of the quality of someone's adaptive behavior requires clinical judgment as well. Certain skills are important to adaptive behavior, such as:

Evidence that the limitations became apparent in childhood[]

This third condition is used to distinguish it from dementing conditions such as Alzheimer's disease or traumatic injuries that damaged the brain.


Psychosocial intellectual disability[]

The lack of mental stimulation and other forms of parental neglect during the early years of development can have severe long-term consequences on intellectual performance.

Genetic and inborn causes[]

Down syndrome, fetal alcohol syndrome and Fragile X syndrome are the three most common inborn causes. However, doctors have found many other causes, both genetic and other. The most common are:

Genetic conditions may be inherited from parents or appear de novo (i.e., due to variances in genes combining/interacting).

X-linked intellectual disability[]

Main article: X-linked intellectual disability

Other genetic conditions include:

Development during pregnancy[]

Intellectual disability can be caused by agents that affect how the fetus develops. For example, there may be a problem with the way the fetus's cells divide as it grows.

Drinking alcohol (see fetal alcohol syndrome) or experiencing infections like like rubella during pregnancy may also cause intellectual disability in the fetus.

Birth complications[]

Complications during labor and birth, such as lack of oxygen to the brain, could result in brain damage and subsequent intellectual disability.

Illness complications[]

Complications from diseases like whooping cough, measles, or meningitis can cause intellectual disability. This is especially likely if medical care is delayed or inadequate.

Exposure to poisons[]

Exposure to poisons like lead or mercury may also affect mental ability.

Nutritional deficiency[]

  • Iodine deficiency, affecting approximately 2 billion people worldwide, is the leading preventable cause of intellectual disability in areas of the developing world where iodine deficiency is endemic. Iodine deficiency also causes goiter, an enlargement of the thyroid gland. Congenital iodine deficiency syndrome may cause mild or more severe intellectual disability. Certain areas of the world due to natural deficiency and governmental inaction are severely affected. India is the most outstanding, with 500 million suffering from deficiency, 54 million from goiter, and 2 million from cretinism. Among other nations affected by iodine deficiency, China and Kazakhstan have begun taking action, whereas Russia has not. [3]
  • Malnutrition is a common cause of reduced intelligence in parts of the world affected by famine, such as Ethiopia. [4]

Treatment and support[]

Intellectual disability is more accurately considered a disability rather than a disease. ID can be distinguished in many ways from mental illness, such as schizophrenia or depression. Currently, there is no "cure" for an established disability, though with appropriate support and teaching, most individuals can learn to do many things.

People with intellectual disabilities may benefit from:

  • Occupational therapy to teach life skills, from tooth-brushing to home ownership
  • Therapy or medication for co-occurring issues, such as speech disorders or mental health concerns

Various agencies provide assistance for people with intellectual and developmental disabilities. They include state-run, for-profit, and non-profit, privately run agencies.

Services for people with intellectual disabilities might include:

  • Fully staffed residential homes
  • Day habilitation programs that approximate schools
  • Workshops wherein people with disabilities can obtain jobs
  • Programs that assist people with developmental disabilities in obtaining jobs in the community
  • Programs that provide support for people with developmental disabilities who have their own apartments
  • Programs that assist them with raising their children

The Burton Blatt Institute at Syracuse University works to advance the civic, economic, and social participation of people with disabilities. There are also many agencies and programs for parents of children with developmental disabilities.

People with developmental disabilities learn throughout their lives and can obtain many new skills even late in life with the help of their families, caregivers, clinicians and the people who coordinate the efforts of all of these people.

Archaic terms[]

Main article: Archaic terms for intellectual disability

Various terms have been invented to describe intellectual disability. Unfortunately, most of these terms have entered popular use as insults. This process is known as the euphemism treadmill.

  • In 1846, S. G. Howe classified people as simpletons, idiots, and fools.[5]
  • In the early 1900s, eugenicist Henry Goddard classified people as idiots (IQ 0-25), imbeciles (IQ 26-50), and morons (IQ-75). All these terms have been used as insults.
  • In the eugenics movement, the terms "feeble-minded" and "mental deficiency" were popular.
  • "Mental retardation" was invented as a newer alternative. It became an insult by the 1960s[6] and is now considered highly derogatory.[7][8]

The euphemism treadmill is likely to continue until societal ableism stops.[9][10]

See also[]

External links[]


  • American Association on Intellectual and Developmental Disabilities (AAIDD). Intellectual Disability: Definition, Classification and Systems of Supports. aka as the “Definition Manual”
  • Emerson E., Hatton C., Bromley, J. & Caine, A. (eds) (1998) Clinical Psychology & People with Intellectual Disabilities. Chichester:Wiley.
  1. 1.0 1.1 Diagnostic Criteria for Intellectual Disabilities: DSM-5 Criteria,
  2. Badano, Jose L., Norimasa Mitsuma, Phil L. Beales, Nicholas Katsanis (September 2006). The Ciliopathies : An Emerging Class of Human Genetic Disorders. Annual Review of Genomics and Human Genetics 7: 125–148.
  3. "In Raising the World’s I.Q., the Secret’s in the Salt", article by Donald G. McNeil, Jr., December 16, 2006, New York Times
  4. "Malnutrition Is Cheating Its Survivors, and Africa’s Future" article in the New York Times by Michael Wines, December 28, 2006
  5. History of Stigmatizing Names for Intellectual Disabilities,
  6. History of Stigmatizing Names for Intellectual Disabilities Continued,
  7. Simpson, John. CHAPTER 2: WHAT’S IN A NAME?, Supporting Individuals with Intellectual Disabilities & Mental Illnesses. Accessed via BC Campus Open Text
  8. Holder, Daisy. The Disability History Glossary, or The History of Disability Words, Disability History Snapshots
  9. Futshane, Lusanda. Inclusive language: outrunning the euphemism treadmill, League Digital
  10. Peterson, Ashley. What Is… the Euphemism Treadmill, Mental Health @ Home