A suite of behaviors characterized by radical mood swings, from euphoria to depression. Two major, distinct modes are recognized: bipolar affective disorder (BPAD, aka manic depressive illness), and unipolar affective disorder (depression without mania). Onset in the first case is usually the second decade, and in the third decade for the latter. Affected individuals with a family history apparently respond better to lithium than do sporadic cases. MZ:DZ twin concordance rates of 57%:14% (see twins and concordant trait), and studies of adoptees correlated with biological parents, suggest some contribution of genetic factors, but with incomplete penetrance. MAFD is an ambiguous multifactorial disorder. Families with affected individuals tend to show co-morbidity with schizoaffective disorder, alcoholism, and/or anorexia nervosa. The possibility of genetic anticipation as an additional mechanism has been proposed. Early linkage studies reporting genes on chromosomes 11 and X (MADF2) have been retracted; other current factors map to 18p.I.