The term “mental retardation” carries decades of clinical history, legal significance, and ongoing debate. Even as language has shifted and newer classifications have emerged, understanding its original definitions remains essential for anyone studying psychology, special education, or clinical practice. What exactly did it mean – and how was it measured? Three major frameworks shaped how this condition was understood: the ICD-10, the DSM-IV-TR, and the AAMD (American Association on Mental Deficiency). Each shares a common core: significantly below-average intelligence combined with deficits in adaptive behavior, both appearing before adulthood.
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What is mental retardation?
Across all major diagnostic systems, mental retardation has been defined by two essential criteria: sub-average intellectual functioning and deficits in adaptive behavior, with onset during the developmental period – generally before age 18.
The ICD-10 classifies mental retardation under codes F70-F79 and defines it as a condition of arrested or incomplete development of the mind, particularly characterized by impairment of skills that emerge during the developmental period – including cognitive, language, motor, and social abilities. The ICD-10 explicitly notes that the condition can occur with or without any other mental or physical condition, which is an important clinical distinction.
The DSM-IV-TR (the version preceding DSM-5) similarly required an IQ score below 70, significant deficits in adaptive functioning in at least two skill areas, and onset before age 18. The DSM-5 later replaced the term with “intellectual disability (intellectual developmental disorder)” and shifted emphasis toward adaptive functioning over strict IQ cutoffs – but for clinical and academic purposes, the DSM-IV-TR framework remains widely referenced.
The AAMD – now known as the American Association on Intellectual and Developmental Disabilities (AAIDD) – was the first body to formally include adaptive behavior as a diagnostic criterion, doing so in its 1959 manual. Its definition states that mental retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior, expressed across conceptual, social, and practical adaptive skills, originating before age 18.
Sub-average intelligence explained
The phrase “sub-average intelligence” has a precise meaning in the context of mental retardation: an IQ score below 70, which places an individual approximately two standard deviations below the population mean.
How IQ is calculated
The original IQ formula was developed by Lewis Terman at Stanford University in 1916, building on the earlier work of Alfred Binet and William Stern. The formula is:
IQ = (Mental Age ÷ Chronological Age) × 100
Mental age refers to the level at which an individual performs intellectually compared to the average performance of their age group. Chronological age is simply their actual age in years. A child performing exactly at the expected level for their age would score IQ = 100. A child performing below that level scores below 100; one performing above it scores higher.
For example, a 10-year-old child who performs at the level typical of an 8-year-old would have an IQ of (8 ÷ 10) × 100 = 80. This places them below average, but not below the clinical threshold of 70. If that same child performed at the level of a 7-year-old, their IQ would be 70 – right at the diagnostic boundary.
The significance of the IQ 70 threshold
On a standard IQ distribution, the mean is 100 and one standard deviation is 15 points. A score of 70 therefore falls two standard deviations below the mean. Modern IQ tests are designed so that roughly 2% of the population scores below 70, and about two-thirds score between 85 and 115. Falling below 70 does not by itself confirm a diagnosis – it must be accompanied by significant deficits in adaptive behavior and onset during the developmental period.
It’s also worth noting that while the ratio IQ formula (MA/CA × 100) was widely used historically, modern tests now use deviation IQ scoring, which compares an individual’s performance directly against a normed sample of same-age peers. The underlying clinical threshold of IQ 70 remains consistent across both approaches.
Deficits in adaptive behavior
IQ scores alone have never been sufficient for a diagnosis of mental retardation. The second core criterion – deficits in adaptive behavior – is equally essential. Adaptive behavior refers to the practical, social, and conceptual skills a person learns and uses in daily life.
The three determinants: maturation, learning, and social adjustment
The AAMD’s 1961 manual first framed adaptive behavior deficits around three key determinants:
- Maturation – the rate at which a child develops basic self-help skills during infancy and early childhood, such as walking, feeding oneself, and toilet training. Delays in these milestones, relative to age and cultural norms, indicate adaptive behavior deficits.
- Learning – the ability to acquire academic skills, generalize knowledge, and apply learning to new situations. Children with mental retardation typically struggle to meet grade-level expectations and may need specialized instructional approaches.
- Social adjustment – the capacity to meet social and community standards of behavior. This includes maintaining employment, following social rules, managing interpersonal relationships, and assuming age-appropriate responsibilities.
This framework, reiterated in subsequent AAMD revisions, described adaptive behavior limitations as significant difficulties in meeting the standards of maturation, learning, personal independence, or social maturity expected for one’s age and cultural group.
What adaptive behavior looks like in practice
The AAIDD identifies three broad domains of adaptive behavior: conceptual skills (language, literacy, money and time concepts, self-direction), social skills (interpersonal relationships, social responsibility, following rules, self-esteem), and practical skills (self-care, household management, safety, occupational skills). A deficit in one or more of these domains – confirmed by standardized assessment tools such as the Vineland Adaptive Behavior Scales – is required alongside a low IQ score for a diagnosis to be made.
It is also important to assess adaptive behavior in cultural context. The WHO ICD working group notes that adaptive behavior limitations must be understood relative to the developmental and sociocultural standards expected of an individual’s peer group – not applied uniformly across all populations regardless of context.
The terminology debate
The term “mental retardation” has a specific and traceable history. It was formally introduced by the American Association on Mental Retardation in 1961 and adopted shortly after by the American Psychiatric Association in its DSM classification system. At the time, it replaced older, more overtly stigmatizing terms such as “idiocy,” “imbecility,” “feeblemindedness,” and “moron.”
Over decades, however, “mental retardation” itself became culturally stigmatized. This triggered a gradual shift toward alternative terms. In the United Kingdom, the preferred clinical term became “learning disability” (replacing earlier terms like “mental handicap” and “mental subnormality”). In international academic and advocacy contexts, “intellectual disability” became the leading replacement. The AAMD itself renamed to the American Association on Intellectual and Developmental Disabilities (AAIDD) in 2007.
In clinical classifications, this shift is now complete. The ICD-11 replaced “mental retardation” with “disorders of intellectual development,” while the DSM-5 (2013) uses “intellectual disability (intellectual developmental disorder).” Both dropped IQ cutoffs as the primary diagnostic criterion in favor of a more comprehensive evaluation of adaptive functioning.
So why does “mental retardation” still appear in clinical, educational, and legal texts? The answer is largely administrative and legal continuity. Many government aid programs, insurance systems, and educational frameworks still reference “mental retardation” because these categories were defined under older legislation. In the United States, for example, some special education categories still use the older terminology under federal disability law. The term persists where “intellectual disability” is not yet specifically covered – even while clinicians increasingly prefer the newer language.
The debate is not merely semantic. It reflects a deeper tension between clinical precision (which requires stable, codified terminology for diagnosis, billing, and research) and social dignity (which demands that language not harm the people it describes). Both concerns are legitimate – which is why understanding both the historical and contemporary terminology matters in any serious study of this field.
What do you think? Given that terms like “mental retardation” have been replaced in official classifications, should older clinical and legal documents be retroactively updated – or does preserving the original terminology serve an important historical and administrative function? And to what extent should cultural and linguistic context shape how adaptive behavior deficits are assessed and diagnosed?
References
- https://www.icd10data.com/ICD10CM/Codes/F01-F99/F70-F79
- https://www.psychiatry.org/patients-families/intellectual-disability/what-is-intellectual-disability
- https://www.aaidd.org/intellectual-disability/definition
- https://www.britannica.com/science/IQ
- https://en.wikipedia.org/wiki/Intelligence_quotient
- https://www.cogn-iq.org/learn/theory/ratio-iq/
- https://nap.nationalacademies.org/read/10295/chapter/6
- https://pmc.ncbi.nlm.nih.gov/articles/PMC3188762/
- https://en.wikipedia.org/wiki/Intellectual_disability
- https://primarycarenotebook.com/pages/paediatrics/mental-retardation-term-that-has-been-replaced-by-intellectual-developmental-disorder
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