Signs of major depression include sadness, feelings of worthlessness, guilt, social withdrawal, and loss of sleep, appetite, sexual desire, or interest in activities.

With severe depression may come slow speech, difficulty in sitting still, inattention to personal appearance, and pervasive feelings of hopelessness and anxiety, as well as suicidal thoughts and feelings. Major depression is one of the most common psychological disorders and may affect about 16% of the population at some time during their lifetimes.

Two types of depression are usually distinguished: unipolar and bipolar. In bipolar depression, a manic mood in which the individual becomes extremely talkative, distractible, seductive, and/or active occurs along with episodes of extreme depression. In unipolar depression, a manic phase is not present.