The most widely used definition of mental retardation (MR) in the United States is:

Mental retardation is a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before age 18.

This definition is from the American Association of Mental Retardation (AAMR), which has had responsibility for defining mental retardation since 1921 (Luckasson & colleagues, 2002).

Intellectual functioning is generally assessed by means of a standardized intelligence test, resulting in an intelligence quotient (IQ). The IQ must be below 70 or 75 for a person to be considered mentally retarded. Adaptive behavior or skills include conceptual (e.g., receptive and expressive language), social (e.g., interpersonal skills), and practical (e.g., personal self-help skills). Adaptive ...

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