Depressive disorders are common and approximately 6-10 percent of the population will experience a depressive episode in any given year. More women than men are affected (2:1) with some estimating that as many as one in five women (i.e. 20 percent) will experience an episode of depression during any given year. All races and socio-economic classes are affected equally, but it is possible that clinicians may underdiagnose depression and overdiagnose schizophrenia in patients from different racial and cultural backgrounds to themselves.

The average age for a first diagnosed episode of major depression is about 40 years, while that for bipolar disorder is 30. Fifty percent of patients have onset between ages 20-50 years. Depression can begin in childhood or in later life, but this is less common and it tends to present differently in different age groups (e.g. childhood (2%) - apathy; adolescence (5%) - behavioural problems; elderly (25 to 50%) - physical complaints).

Depressive disorders are more likely in those individuals who are socially isolated and have no close interpersonal relationships or who are divorced or separated.