Clinical<unk>  is the leading cause of disability worldwide, and in 2000 was the fourth leading contributor to the global burden of disease (DALYs); it is also an important risk factor for suicide. It is understandable, then, that clinical<unk>  is thought to be a pathology -- a major dysfunction of the brain. Yet the epidemiology of clinical<unk>  is something of an outlier when compared to the epidemiology of major dysfunctions of other organs like the heart, liver, lungs and kidneys. In most cases, rates of organ dysfunction increase with age, with low rates in adolescents and young adults, and the highest rates in the elderly. These patterns are consistent with<unk>  theories of aging which posit that selection against dysfunctional traits decreases with age (because there is a decreasing probability of surviving to later ages).
In contrast to these patterns, prevalence of clinical<unk>  is high in all age categories, including otherwise healthy adolescents and young adults. In one study of the US population, for example, the 12 month prevalence for a major<unk>  episode was highest in the youngest age category (15-24 year olds). The high prevalence of<unk>  is also an outlier when compared to the prevalence of major mental retardation, autism, and schizophrenia, all with prevalence rates about one tenth that of<unk> , or less.
The common occurrence and persistence of a trait like clinical<unk>  with such negative effects early in life is difficult to explain. (Rates of infectious disease are high in young people, of course, but clinical<unk>  is not thought to be caused by an infection.)<unk>  psychology suggests ways in which behaviour and mental states, including seemingly harmful states such as<unk> , may be adaptations to recurring reproductive problems faced by our ancestors, actually having improved the fitness of either the individual or their relatives. It has been argued, for example, that Abraham Lincoln's life-long<unk>  was a source of insight and strength. Some even suggest that "we aren’t designed to have happiness as our natural default" and so a state of<unk>  is the<unk>  norm.
The following hypotheses attempt to identify a benefit of<unk>  that outweighs its obvious costs. All take as their starting point the fact that one of the most potent, well-established causes of major<unk>  is a severe negative life event.
Evolutionary approaches to depression
Personality disorder interventions
Suicide psychotherapy
Delusional disorder medical therapy
Suicide Pharmacotherapy
Drug rehabilitation
Anhedonia medical therapy
Mind–body intervention