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subaverage general intellectual functioning that originates during the developmental period and is associated with impairment in adaptive behavior. The American Association on Mental Deficiency lists eight medical classifications and five psychological classifications; the latter five replace the three former classifications of moron, imbecile, and idiot. Mental retardation classification requires assignment of an index for performance relative to a person's peers on two interrelated criteria: measured intelligence (IQ) and overall socioadaptive behavior (a judgmental rating of the person's relative level of performance in school, at work, at home, and in the community). In general an IQ of 70 or less indicates mental retardation (mild = 50/55-70; moderate = 35/40-50/55; severe = 20/25-35/40; profound = below 20/25); an IQ of 70-85 signifies borderline intellectual functioning.
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Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005