Evolutionary approaches to depression
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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 is much more common than schizophrenia or autism, and its prevalence does not increase with age the way dementia and other 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.
2 Psychic pain hypothesis
3 Behavioral shutdown model
4 Analytical rumination hypothesis
5 Possibilities of depression as a dysregulated adaptation
6 Rank theory
7 Social risk hypothesis
8 Honest signaling theory
9 Bargaining theory
10 Social navigation or niche change theory
11 Prevention of infection
12 The third ventricle hypothesis
14 See also
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. 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.
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.
Psychic pain hypothesis
One reason depression is thought to be a pathology is that it causes so much psychic pain and distress. However, physical pain is also very distressful, yet it has an evolved function: to inform the organism that it is suffering damage, to motivate it to withdraw from the source of damage, and to learn to avoid such damage-causing circumstances in the future. Sadness is also distressing, yet is widely believed to be an evolved adaptation. In fact, perhaps the most influential evolutionary view is that most cases of depression are simply particularly intense cases of sadness in response to adversity, such as the loss of a loved one.
According to the psychic pain hypothesis, depression is analogous to physical pain in that it informs the sufferer that current circumstances, such as the loss of a friend, are imposing a threat to biological fitness. It motivates the sufferer to cease activities that led to the costly situation, if possible, and it causes him or her to learn to avoid similar circumstances in the future. Proponents of this view tend to focus on low mood, and regard clinical depression as a dysfunctional extreme of low mood—and not as a unique set of characteristics that are physiologically distanced from regular depressed mood.
Alongside the absence of pleasure, other noticeable changes include psychomotor retardation, disrupted patterns of sleeping and feeding, a loss of sex drive and motivation—which are all also characteristics of the body's reaction to actual physical pain. In depressed people there is an increased activity in the regions of the cortex involved with the perception of pain, such as the anterior cingulate cortex and the left prefrontal cortex. This activity allows the cortex to manifest an abstract negative thought as a true physical stressor to the rest of the brain.
Behavioral shutdown model
The behavioral shutdown model states that if an organism faces more risk or expenditure than reward from activities, the best evolutionary strategy may be to withdraw from them. This model proposes that emotional pain, like physical pain, serves a useful adaptive purpose. Negative emotions like disappointment, sadness, grief, fear, anxiety, anger, and guilt are described as "evolved strategies that allow for the identification and avoidance of specific problems, especially in the social domain." Depression is characteristically associated with anhedonia and lack of energy, and those experiencing it are risk-aversive and perceive more negative and pessimistic outcomes because they are focused on preventing further loss. Although the model views depression as an adaptive response, it does not suggest that it is beneficial by the standards of current society; but it does suggest that many approaches to depression treat symptoms rather than causes, and underlying social problems need to be addressed.
A related phenomenon to the behavioral shutdown model is learned helplessness. In animal subjects, a loss of control or predictability in the subject's experiences results in a condition similar to clinical depression in humans. That is to say, if uncontrollable and unstoppable stressors are repeated for long enough, a rat subject will adopt a learned helplessness, which shares a number of behavioral and psychological features with human depression. The subject will not attempt to cope with problems, even when placed in a stressor-free novel environment. Should their rare attem