At any point in time on this planet, almost two and a half times as many persons are depressed as are demented. Counting all variations on the theme, about 10% of men and 20% of women are at lifetime risk for experiencing a depressive illness. Based on numbers of persons affected, the World Health Organization estimates that depressive illnesses are the fourth highest cause of medical disability and premature death worldwide in years of illness, treatments required, lost productivity during episodes, and death rates.

Ten to 15% of depressed persons die by suicide. This rate translates into about one million persons annually, worldwide, and 30 000–90 000 persons annually in the USA. The first figure is the accepted count and the latter figure is estimated from analyses that acknowledge underreporting. In the USA, suicide is the 11th leading cause of death. Most persons over age 50 have known at least one individual who has committed suicide. Among depressed persons, sufferers of melancholia have the highest suicide rates.

Depressive illness is increasing in frequency among persons born closer to the present (a period effect), and first episodes are occurring at younger ages (a cohort effect).