Clinical depression (also called major depressive disorder, or sometimes unipolar when compared with bipolar disorder) is a state of intense sadness, melancholia or despair that has advanced to the point of being disruptive to an individual's social functioning and/or activities of daily living. Although a low mood or state of dejection that does not affect functioning is often colloquially referred to as depression, clinical depression is a clinical diagnosis and may be different from the everyday meaning of "being depressed". Many people identify the feeling of being depressed as "being blue", "feeling sad for no reason", or "having no motivation to do anything".
One suffering from depression may feel tired, sad, irritable, lazy, unmotivated, and apathetic. Clinical depression is generally acknowledged to be more serious than normal depressed feelings. It often leads to constant negative thinking and often substance abuse. The modern idea of depression appears similar to the much older concept of melancholia. The name melancholia derives from "black bile", one of the "four humours" postulated by Galen.
Clinical depression was originally considered to be a chemical imbalance in transmitters in the brain, a theory based on observations made in the 1950s of the effects of reserpine and isoniazid in altering monoamine neurotransmitter levels and affecting depressive symptoms. Since these suggestions, many other causes for clinical depression have been proposed. Clinical depression affects about 16% of the population on at least one occasion in their lives. In some countries, such as Australia, one in four women and one in eight men will suffer from depression. The mean age of onset, from a number of studies, is in the late 20s. About twice as many females as males report or receive treatment for clinical depression, though this imbalance is shrinking over the course of recent history; this difference seems to completely disappear after the age of 50 - 55, when most females have passed the end of menopause. Clinical depression is currently the leading cause of disability in North America as well as other countries, and is expected to become the second leading cause of disability worldwide (after heart disease) by the year 2020, according to the World Health Organization.
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