Brief review of the literature reveals that the evidence for a genetic factor in the etiology of schizophrenia is convincing. The evidence suggest that a tendency toward development of the disease is inherited but that its appearance and clinical features depend upon environmental and experiential factors. The question of the mechanism whereby the genes responsible for schizophrenia maintain their levels in human populations in the face of the obvious selective disadvantage conferred by the disease is as yet unanswered. The work of Heston and Karlsson suggests that a selective advantage may accrue to the heterozygote, a mechanism explainable in terms of the concept of genetic polymorphism, as described by E. B. Ford.