Thus, the term mood disorder is generally used to describe conditions in which abnormalities of emotional state are the core symptoms. The lifetime prevalence of bipolar disorder is one to two percent. In contrast, bipolar disorder affects men and women equally. The age of onset of unipolar depression appears to be decreasing, and the incidence of depression appears to be increasing with successive birth cohorts in several Western countries, including the United States, Canada, Germany, and Sweden. This disturbing trend cannot be explained by artifacts of diagnosis, recall, or reporting.
Mood disorders involve serious morbidity and a substantial risk of death.
Patients with mood disorders also have a higher mortality than age-matched control subjects because of a higher incidence of accidents and other illnesses. In a 16-year prospective study of a general population sample, patients with mood disorders experienced 1.5 times the number of deaths that would be expected on the basis of rates for a large reference population. It has long been known that mood disorders run in families. This pattern has been established by studies that compared illness rates in relatives of depressed patients with rates in relatives of normal control subjects. For example, the prevalence of mood disorders in first-degree relatives of depressed individuals is at least twice that in relatives of randomly selected individuals.