Genetic epidemiology of an institutionalized cohort of mental retardates.

By criterion scaling and principal component analysis of performance, social class, symptoms, institutionalization, and medical history, a cohort of mental defectives has been divided into medical, biological, and sociofamilial categories. This division, corresponding to differences in etiology and severity, reveals changing patterns of admission and provides evidence that male excess is not primarily due to sex linkage. The incidence of mental retardation increases about 5% with first-cousin marriage, in agreement with other studies. The decline of IQ with inbreeding appears to be due entirely to rare recessive genes, not to dominance deviations of polygenes.