Genetic epidemiology of an institutionalized cohort of mental retardates

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Abstract
By criterion scaling and principal component analysis of performance, social class, symptoms, institutionalization and medical history, 702 human mental defectives were 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 1st-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.