Intellectual Disability - Classification

Classification of intellectual disability provides some clarity of severity of disability and the level of support that will be required in the educational system and beyond. Intelligence, measured as IQ, and adaptive behavior scales are both important.
Keep potential confounding factors in mind. In everyday life, children with motor problems or auditory or visual impairments might perform lower than their measured IQ would predict. Similarly, receptive or expressive language or motor disabilities may result in IQ scores that underestimate their intellectual potential. Adaptive measures, typically assessed by interview, may vary depending on the reliability of the informant.
IQ score categories from the ICD-9 [National: 2007] classification and the DSM-IV [American: 2000] classification (in parentheses) are included below; typically the definition of mental retardation requires that a child be below the population average by at least two standard deviations on measure of intelligence.
  • Borderline Intellectual Functioning - IQ=67-85 (71-84, DSM-IV).
  • Mild MR - IQ=52-68, ICD9 (50-55 to 70, DSM-IV)
    • Affected children are able to speak and to learn some social skills.
    • They can usually be expected to care for themselves as adults, with some guidance.
  • Moderate MR - IQ=36-51, ICD9 (35-40 to 50-55, DSM-IV)
    • Affected children, although they are able to learn some language, usually have poor social skills. They will be able to achieve in school to about the elementary school level. Because their early motor milestones are usually attained in the normal range, children at this IQ level and above tend to be diagnosed around the preschool period. However, language development and achievement of activities of daily living and social skills are often delayed.
    • They will generally need complete supervision as an adult, often in a group home setting. They may be capable of unskilled occupations in a supported-employment setting.
  • Severe MR - IQ=20-35, ICD-9 (20-25 to 35-40, DSM-IV)
    • Children with severe and profound MR are often diagnosed very early because acquisition of even the earliest motor milestones are delayed.
    • Affected children will be able to learn a few words and a few self-help skills, but will need a protected environment as an adult. A living situation in a group home with increased support will generally be possible.
  • Profound MR - IQ=19 and below, ICD-9 (less than 20-25, DSM-IV)
    • Affected children will generally need full care as adults, often in a nursing home environment.


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Helpful Articles

Ruth Luckasson, Robert L. Schalock, Deborah M. Spitalnik, Scott Spreat, Marc Tassé, Martha E. Snell, David L. Coulter, Sharon A. Borthwick-Duffy, Alison Alya Reeve, Wil H.E. Buntinx, and Ellis (Pat) M. Craig.
Mental Retardation: Definition, Classification and Systems of Supports.
Washington DC: AAIDD; 2002. 0-940898-81-0

Authors & Reviewers

Initial publication: September 2008; last update/revision: October 2008
Current Authors and Reviewers:
Author: Lynne M. Kerr, MD, PhD

Page Bibliography

American Psychiatric Association.
Diagnostic and Statistical Manual of Mental Disorders DSM-IV-TR (Text Revision).
4th edition (June 2000) ed. Washington, DC: American Psychiatric Association; 2000. 0890420254

National Center for Health Statistics (NCHS) and the Centers for Medicare and Medicaid Services .
International Classification of Diseases, Ninth Revision, Clinical Modification.
10th revision ed. Various Publishers; 2007.