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  • Updated 08.10.2023
  • Released 05.03.1994
  • Expires For CME 08.10.2026

Intellectual disability



In recent years, the age of onset criteria for intellectual disability has been augmented to include disabilities that originate before the age of 22 years, whereas previously, it originated before the age of 18. This broadened inclusion criteria highlights the importance of a newer understanding that neurologic development continues into our 20s, and critical areas of growth and development continue into early adulthood. Prevalence rates have not been impacted due to this change, as most diagnoses continue to be made in childhood. Additionally, the COVID-19 pandemic made worldwide impact and raised new concerns about the social implications of future pandemics and health crises for those with intellectual disability.

Historical note and terminology

Clear examples of people with intellectual disability have been recorded since biblical times. During the medieval period, there was legal and medical interest in differentiating between idiots and lunatics. A 13th-century law distinguished between those unfit to inherit because of innate incapacity and those who could be temporarily deprived of their inheritance while judged "out of their mind." During the 15th and 16th centuries, courts evaluated idiocy by competence in numeracy and social skills. Labels used were "idiot," "fool," "non compos mentis," or "an innocent." The evidence was that they could not perform everyday tasks such as counting to 20, name their parents or neighbors, clothe themselves, or go out unaided. They were not generally considered a social threat, as opposed to lunatics, where a danger to self or others was a factor in judging insanity. In addition, lunacy was considered a possible temporary condition, whereas idiots were said to be "incurably and naturally damaged" (75). In 1614, Montalto in Florence published a major medical textbook called Archipathologia. His chapter entitled "on Loss of Mental Faculties and Simple Mindedness" (translation from Latin), distinguishes between intellectual disability, mental illness, dementia, and delirium. He stated that these conditions originated in the brain. He also recognized that intellectual disability could arise from birth defects and postnatal causes, including brain damage at birth.

By the late 19th and 20th centuries, with the coming of universal school education, recognition and assessment of intellectual disability was required for educational purposes, and more formal testing of individuals began. Categories used professionally at one time included idiocy, imbecility, feeble-mindedness, simpleton, and moron. The terms used most commonly to describe intellectual disability in the twentieth century included mental retardation, mental deficiency, mental subnormality, amentia, and oligophrenia.

Intellectual disability is currently defined by the American Association on Intellectual and Developmental Disabilities (formerly American Association on Mental Retardation [AAMR]) as “a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical skills. This disability originates before the age of 22” (02). In 2007, this organization renamed itself, as well as renaming this disability from the prior term “mental retardation” (78) to better conform to current constructs of disability. This disability is also defined similarly by the American Psychiatric Association in its Diagnostic and Statistical Manual 5 as “a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains.” Additionally, the DSM-5 uses the term “intellectual developmental disorder” as the equivalent term for “intellectual disability” because the upcoming ICD-11 will likely use the term “intellectual developmental disorder” to indicate involvement of impaired brain functioning early in life.

Moreover, a federal statute in the United States (Public Law 111-256, Rosa’s Law) replaces the term mental retardation with intellectual disability; thus, intellectual disability is the term used in law and public policy in the United States, whereas “learning disability” is used in the United Kingdom.

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