Major depression is when five or more symptoms of depression are present for at least 2 weeks. These symptoms include feeling sad, hopeless, worthless, or pessimistic. In addition, people with major depression often have behavior changes, such as new eating and sleeping patterns. Major depression increases a person's risk of suicide.

The exact cause of depression is not known. Many researchers believe it is caused by chemical imbalances in the brain, which may be hereditary or caused by events in a person's life.

Some types of depression seem to run in families, but depression can also occur in people who have no family history of the illness. Stressful life changes or events can trigger depression in some people. Usually, a combination of factors is involved.

Each year, more than 18 million Americans -- men and women of all ages, races, and economic levels -- have depression. It occurs more often in women.

Women are especially vulnerable to depression after giving birth. This is a result of the hormonal and physical changes. While new mothers commonly experience temporary "blues," depression that lasts longer than 2-3 weeks is not normal and requires treatment.

Major depression can occur in children and teenagers, and they can also benefit from treatment.

See also: Adolescent depression

• Trouble sleeping or excessive sleeping
• A dramatic change in appetite, often with weight gain or loss
• Fatigue and lack of energy
• Feelings of worthlessness, self-hate, and inappropriate guilt
• Extreme difficulty concentrating
• Agitation, restlessness, and irritability
• Inactivity and withdrawal from usual activities, a loss of interest or pleasure in activities that were once enjoyed (such sex)
• Feelings of hopelessness and helplessness
• Thoughts of death or suicide

Depression can appear as anger and discouragement rather than feelings of hopelessness and helplessness. If depression is very severe, it may be accompanied by psychotic symptoms, such as hallucinations and delusions. These are usually consistent with the depressed mood, and may focus on themes of guilt, personal inadequacy, or disease.

Major depression is diagnosed if the person reports having five or more depressive symptoms for at least 2 weeks. Beck's Depression Scale Inventory or other screening tests for depression can be helpful in diagnosing depression.

Medical causes that can cause symptoms of depression should also be ruled out before making the diagnosis of depression.