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The cause of mood disorders cannot be narrowed down to a single factor. However, it appears that brain chemistry plays a major role. Brain chemicals called neurotransmitters convey messages between the nerves.  Abnormalities in the regulation of these neurotransmitters, particularly norepinephrine, serotonin and dopamine, are believed to cause alterations in mood.

Mood disorders also appear to be linked to genetics, and researchers are making great strides in identifying the genetic links involved. Individuals who have relatives with a history of a mood disorder are at a higher risk of developing either the same or a different mood disorder. This is especially true for first-degree relatives (e.g., parents, children, siblings). Some researchers believe that some people inherit a tendency to develop mood disorders, which may then be triggered by environmental factors (e.g., stressful life events, disturbances in seasonal or circadian rhythm).

Gender is another major risk factor for mood disorders, particularly depressive disorders. According to the National Institute of Mental Health (NIMH), women have nearly twice as much risk of developing major depression as men. Many mental health professionals believe that hormones involved in pregnancy and miscarriage and hormone changes during the menstrual cycle, postpartum period, premenopause and menopause may lead to a major depressive episode.