Relative importance of genetic and nongenetic etiologies in idiopathic mental retardation: estimates based on analysis of medical histories

A method of analyzing subgroups of mental retardates for proportions of cases caused by nongenetic brain damage is presented here and applied to 490 Israeli retardates. Among mild retardates in simplex and multiplex families with normal unrelated parents, most of the cases were caused by nongenetic brain damage and only a minority were caused by polygenic heredity. Among severe retardates in multiplex families with normal unrelated parents, nongenetic brain damage is probably a more important cause than is homozygosity. In all groups except two (mild retardates with normal consanguineous parents, and severe retardates with retarded parents) the contribution of nongenetic brain damage was surprisingly large.