Epidemiology of severe mental retardation in children: community studies.

Twenty-seven community studies of severe mental retardation, defined as IQ less than 50, were reviewed. The prevalence rate of this condition was about 4 per 1000 in older children; the rate was somewhat higher in males but did not vary by social class. About one-half of severely retarded children had significant associated handicaps. The cause of most cases of severe mental retardation is not known, but Down's syndrome accounted for one-sixth to one-third of cases and a small percentage were due to other chromosomal abnormalities, metabolic diseases, or infection.