Mental retardation is characterized by
significantly subaverage levels of general
intellectual functioning. Recent definitions
have also emphasized such characteristics as its onset in the childhood years and impairments in adaptive behavior, but most researchers and professionals who work with the retarded continue to rely on subnormal intelligence as the key definitional feature. Developers of the standardized tests measuring intelligence quotient, or IQ, set the average IQ at 100. The standard deviation from this average is 15.
Significantly subaverage
intellectual function, or mental retardation, is defined as being more than two standard deviations below 100Ñthat is, 70 or below.
Retardation affects from 2% to 3% of the population, although some experts argue that only about 1% should receive the diagnosis. The large majority of retarded persons fall in the mild range of retardation (IQ = 55-70), and smaller numbers are moderately (IQ = 40-54), severely (IQ = 25-39), and profoundly (IQ = 0-24) retarded. More males than females are retarded, and retardation can coexist with psychological disorders (for example, autism and schizophrenia) and physical disabilities (for example, cerebral palsy and spina bifida).