The term Intellectual Disability (ID), commonly used in Europe, is now becoming preferred to the previous one Mental Retardation (MR), although both are still used interchangeably. Developmental disability is a legal term defined by the Developmental Disabilities Assistance and Bill of Rights Amendments of 1996, and is used in statutes referring to entitlements. Its requirements are: The condition is due to mental and/or physical impairment, is manifested prior to age 22, is likely to be lifelong, results in substantial functional limitations in three or more major life activities and in need for lifelong services or supports.


ID does not denote an illness or a single disorder entity but a behavioral syndrome of variable etiology characterized by intellectual and adaptive functioning below level expected for the person's age, education and socio-cultural context. In fact, strictly speaking, it is not a mental disorder like a mood or psychotic disorder. Formal definitions of ID have evolved over time. The modern ones are tri-factorial, requiring impairment in (1) intellectual functioning, (2) adaptive functioning, and (3) onset before age 18. In the US, the standard definition is of the Diagnostic and Statistical Manual of Mental Disorders, fourth edition, Text Revision (DSM-IV-TR) (American Psychiatric Association 2000).


DSM-IV-TR Diagnostic Criteria


Mental Retardation


  1. Significantly subaverage intellectual functioning: An IQ of approximately 70 or below on an individually administered IQ test (for infants, a clinical judgment of significantly subaverage intellectual functioning).

  2. Concurrent deficits or impairments in present adaptive functioning (i.e., the person's effectiveness in meeting the standards expected for his or her age by his or her cultural group) in at least two of the following areas: Communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.

  3. The onset is before age 18 years


Code based on degree of severity reflecting level of intellectual impairment:


Mental Retardation has the following levels of severity and codes:

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317Mild Mental RetardationIQ level 50–55 to approximately 70
318.0Moderate Mental retardationIQ level 35–40 to 50–55
318.1Severe Mental RetardationIQ level 20–25 to 35–40
318.2Profound Mental RetardationIQ level below 20 or 25
319Mental Retardation Severity Unspecified: When there is strong presumption of mental retardation but the person's intelligence is untestable by standard tests

(Reprinted, with permission, from the Diagnostic and Statistical Manual of Mental Disorders, 4th edn. Text Revision. Copyright 2000 American Psychiatric Association.)


The second most common definition is of the American Association on MR. It also requires onset prior to age 18 and presence of significant limitations in (A) intellectual functioning (defined as an IQ score of 2 SD or more below the mean of an individually administered assessment instrument) and (B) adaptive behavior (in conceptual, social, and practical skills). For the purpose of either ...

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