A friend tells me he was recently diagnosed with "mild" depression. I told him that you are either depressed or not depressed. Are there real differences in the severity of depression? If so, what are they?

For the answer to this, let's go to The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). It's the diagnostic "bible" for mental health care professionals. According to DSM-IV, major depressive disorder (depression, for short) is measured along a continuum ranging from mild to moderate to severe.

To be considered depressed according to DSM-IV, a person must experience for at least two weeks either depressed mood or loss of interest and pleasure in nearly all activities. (Children and adolescents may seem irritable rather than sad.) The individual must also experience at least four additional symptoms from a list that includes: changes in appetite, weight, or sleep (such as insomnia or hypersomnia -- sleeping much more than usual); changes in psychomotor activity (seeming "speeded up," agitated, or "slowed down"); decreased energy; feelings of worthlessness or guilt; difficulty thinking, concentrating, or making decisions; recurrent thoughts of death; or suicidal thoughts, plans, or attempts.

You may be surprised that someone can be considered clinically depressed even if they don't report experiencing sadness or depressed mood. Instead, they may have lost interest in most things they used to enjoy doing, such as socializing, working, or having sex. (Such loss of interest and pleasure in usual activities is called "anhedonia.")

People with mild, moderate, or severe depression have difficulty functioning at work, at home, or in personal relationships. Those with mild episodes may appear to function normally, but actually are having to struggle or expend more effort than usual to maintain the status quo. People with extreme or severe depression may be unable to work altogether. They may withdraw socially, be unable to maintain personal relationships or to feed or clothe themselves or maintain personal hygiene. Some people (but not all) with severe depression develop psychotic features such as delusions.

There's another type of depressive disorder that might be considered a mild form of depression. If a person experiences depressive symptoms for two years or more, but does not meet the criteria for major depressive disorder, he or she might be diagnosed with dysthymic disorder, or dysthymia