Major depression is the most common psychiatric illness; as many as 1 out of 20 people at some point in their lives may meet criteria for having the illness, according to the standards set out by the Diagnostic and Statistical Manual IV.

Of those, about one-third will recover without any psychological or medical intervention, another third will go on to develop "dysthymia," a chronic low-grade depressive illness, while another third will remain clinically depressed at the end of one year.  Major depression is characterized by feeling sad or empty nearly every day, loss of interest or pleasure in hobbies or activities, either a loss or marked increase in appetite and weight, problems with insomnia or oversleeping, as well as thoughts of guilt, death, or suicide at times. Psychotherapy and antidepressive medications both are helpful with as many as 70% to 80% of persons responding to the use of today's antidepressant medications.

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