A critical examination of the data for and against genetic factors in early infantile autism and childhood schizophrenia is presented. The extreme rareness of both disorders made analysis difficult. No strong evidence exists implicating genetics in the development of childhood psychoses that begin before the age of 5. Family pedigree data fail to support psychogenic transmission because very few siblings of early onset cases are affected. Biological but not genetic etiological agents are more likely. Genetic factors are implicated in the development of psychoses that begin near pubescence and such factors appear to overlap with those for adult schizophrenia. Reevaluation of the minimum age of onset for adult-type schizophrenia is suggested.