Depression can vary widely in severity, and it's normal to have feelings of sadness from time to time. Most everyone experiences reduced self-esteem, pessimistic thoughts, or brief periods of social withdrawal when they fail to achieve something, or when they lose someone important to them.

Treatment of major depression varies greatly depending on the nature and severity of the illness, and the theoretical perspective the psychologist or doctor is working from.

Modern researchers have not yet determined what causes depression, but the mood disorder appears to be related to elevated stress and life changes, and may be influenced by brain chemicals.