THE ROLE OF heredity in the major psychiatric disorders and the incorporation of genetic knowledge into related clinical and biologic research continue to be problematic issues for contemporary psychiatric thought. The purpose of this communication is to outline the current status of genetic knowledge in the affective disorders and to apply this knowledge to recent developments in the phenomenology, etiology, and treatment of these disorders. Four areas will be considered: First, the evidence for a genetic factor in affective illness; second the relative contribution of genetic and nongenetic factors in determining who will manifest an affective disorder ("heritability"); third, genetic contributions to nosology and to the clinical and biological phenomenology of affective illness; and fourth, the mode of genetic transmission of the predisposition to these disorders. Some implications of genetic knowledge for future biologic research in affective disorders will be discussed.

Evidence for a Genetic FactorÌý A genetic hypothesis in