Symptoms and signs often cluster into discrete syndromes that typically recur or, less commonly, persist without remission. Clinical depression and mania, unlike normal emotional reactions, cause marked impairment in physical function, social function, and work capacity. These figures largely represent unipolar major depressive disorder and its variants. Persons born in the 2 decades after World War II have higher rates of depression and suicide, often associated with higher rates of substance abuse, than those born earlier. However, bipolar disorder is somewhat more common in upper socioeconomic classes.
Cultural factors seem to modify the clinical manifestations of mood disorders.
Economic factors, such as unemployment and sudden financial reversals, have been linked to increased suicide rates in men. Heredity is the most important predisposing factor. Polygenic inheritance as a common genetic substrate for bipolar and recurrent unipolar disorders is a more popular hypothesis. What is inherited is unknown. Childhood loss of a parent does not increase a person's risk of developing a mood disorder.