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Evolutionary approaches to depression are attempts by evolutionary psychologists to use the theory of evolution to shed light on the problem of mood disorders. Depression has generally been thought of as dysfunction, but it does not increase with age the way organic dysfunction commonly does. Some researchers have surmised that the disorder may have evolutionary roots, in the same way that others suggest evolutionary contributions to schizophrenia, sickle cell anemia and other disorders. Psychology and psychiatry have not generally embraced evolutionary explanations for behaviors, and the proposed explanations for the evolution of depression remain controversial. == Background == Major depression (also called "major depressive disorder", "clinical depression" or often simply "depression") is a leading cause of disability worldwide, and in 2000 was the fourth leading contributor to the global burden of disease (measured in DALYs); it is also an important risk factor for suicide. It is understandable, then, that clinical depression is thought to be a pathology — a major dysfunction of the brain. 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.〔(【引用サイトリンク】title=Prevalence of Heart Disease --- United States, 2005 )(【引用サイトリンク】title=Prevalence of Chronic Kidney Disease and Associated Risk Factors --- United States, 1999--2004 )〕 These patterns are consistent with evolutionary 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 depression 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 depression episode was highest in the youngest age category (15- to 24-year-olds). The high prevalence of depression 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 depression, or less.〔Epidemiologic Catchment Area, National Comorbidity Study〕 The common occurrence and persistence of a trait like clinical depression with such negative effects early in life is difficult to explain. (Rates of infectious disease are high in young people, of course, but clinical depression is not thought to be caused by an infection.) Evolutionary psychology and its application in evolutionary medicine suggest how behaviour and mental states, including seemingly harmful states such as depression, may have been beneficial adaptations of human ancestors which improved the fitness of individuals or their relatives. It has been argued, for example, that Abraham Lincoln's lifelong depression 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 depression is the evolutionary norm. The following hypotheses attempt to identify a benefit of depression that outweighs its obvious costs. Such hypotheses are not necessarily incompatible with one another and may explain different aspects, causes, and symptoms of depression.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolutionary approaches to depression」の詳細全文を読む スポンサード リンク
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