Dark Side of the Moon

I wrote this paper for bio and thought I’d put it up. Sorry about the formatting, as always. Don’t know why the works cited part freaks out like that :/


Dark Side of the Moon: Depression as an Adaptation

Biologists have long puzzled over the pervasiveness of mood disorders. Conditions like depression have dire, often lethal, consequences. Why would evolution conserve the depressive tendency in such large proportions of the population when it has the potential to kill the afflicted individual? Evidence suggests that depression is not a straightforward pathology but instead an adaptive trait that has become maladaptive in the modern world. Numerous hypotheses in the field of evolutionary psychology attempt to explain how a condition with such high fitness costs could be considered adaptive. Two leading ideas include the analytical rumination hypothesis, which describes depression as a problem-solving mechanism, and the honest signaling hypothesis, which paints depression as a bargaining signal for help from social allies. Both of these hypotheses look at depression as a phenomenon following a traumatic life event and do not adequately explain severe depressive episodes that have seemingly no underlying trigger.

Certain aspects of depression suggest that it may at one time have been an adaptive trait. In the United States, an estimated 17% of people have suffered a major depressive episode in their lifetime (Andrade). As a comparison, less than 1% of the population suffers from schizophrenia, another common mental illness. Moreover, unlike most illnesses that increase in occurrence as people age, depression can occur throughout a person’s lifetime, peaking during the ages of 20 to 30 years. Lastly, depression seems to have a strong heritable component. All of these aspects suggest that depression is not a dysfunctional pathology but instead an adaptation that may have provided an important benefit for our ancestors.

Outside of the clinical context, the concepts of depression and sadness often become conflated. Sadness is simply one aspect of major depression, which includes other symptoms such as low self-esteem and loss of pleasure in former interests (anhedonia). Primitive emotions such as anxiety and anger serve a purpose for the individual experiencing them. Anxiety alerts the individual to a dangerous element and prompts the fight-or-flight response whereas anger prompts the aggression necessary to drive away an attacker. Evolutionary psychologists predict that sadness serves a similarly helpful function. What that function is, however, is more difficult to determine.

One clue lies in the analytical rumination hypothesis. This hypothesis proposes that depression is an adaptation developed in order to work through complex problems via obsessive rumination. Due to the prevalence of depression within all cultures and age groups, proponents of this hypothesis suggest that depression is not a pathology at all but a normal psychological function (Thomson). That is, clinical levels of depression do not indicate a disorder but a normal response to stress. It may be useful to compare the depressive response to a fever: fever is a costly and potentially lethal bodily function, but its role in ridding the body of noxious contaminants is worth the expenditure. Thus, it is not categorized as a dysfunction. Similarly, depression keeps an individual isolated and unable to partake in important activities such as sex or child rearing which can have significant effects on fitness. It forces the individual to invest the vast majority of her energy ruminating on a specific problem. The analytical rumination hypothesis proposes that the insights gained from the rumination make up for the socially alienating and painful aspects of depression.

Individuals suffering from depression do not become excellent problem solvers overnight. There are three proposed aspects of depression that promote analytical problem solving: first, the events that trigger depression tend to be complex problems that affect fitness-related goals. Complex problem-solving requires a different mode of thinking–each component of the problem must be first broken down individually, then reintegrated.

Second, the depressive state creates neurological changes that focus the brain on breaking down a complex problem into simpler parts. Research shows that when non-depressed individuals attempt to solve a complex problem, their depressive affect increases during rumination (Andrews). This aspect of rumination can actually appear to impair cognition as it prevents depressed individual from focusing on anything other than their original problem. Depression is an enormous mental resource-suck and this may be the reason why depressed individuals consistently display impaired memory and attention in lab tests (Goodwin). However, a battery of cognitive tests in a lab setting is unlikely to be the kind of complex problem that depression was meant to solve.

Third, over the course of evolutionary time, depression helps people solve the problem that triggered the depressive episode. Proponents of the analytical rumination hypothesis point to the failure of anti-depressants to create long term change as evidence. Many therapists find that while anti-depressants alleviate psychic pain, patients relapse once they are taken off medication if the underlying initial problem is not solved. Conversely, for many patients an episode of major depression terminates once the triggering event has come to a conclusion.

If depression is not meant for solving lab tests, then what types of problems is it meant for? The honest signaling hypothesis models the syndrome as a social bargaining tool employed by individuals who need to re-negotiate some aspect of the social contract. Individual human fitness relies a great degree on complex social interactions. As mentioned above, the emotion of anger can propel an individual to demonstrate aggression in a situation where she feels threatened. However, if an individual is up against a much larger individual or a group of individuals, then aggression may not be the best tactic (Hagen 96). In this case, she must signal to others in her social group that she needs help. The ill health of a depressed individual affects not only her own fitness but also that of her offspring, mate, and kin. Thus, they have a vested interest in coming to her aid. Women are twice as likely to experience major depression as are men, and when social role variables are controlled for, 50 percent more likely. Since women tend to be less physically strong, this provides support for the idea that depression is an alternative to physical aggression.

The difference between honest and dishonest signaling may shed light on depression’s high costs on fitness. An honest signal requires costly resources because it needs to be difficult to mimic. If it were simple to mimic, others would employ it dishonestly in order to take advantage of its benefits and the meaning of the signal would become diluted. The complete withdrawal of the depressed person from her environment signals the direness of her situation to her social allies and compels them to action. This tactic can be compared to that of worker’s strike, where a worker cuts off her stream of income in order to bring about a necessary change (100). Thus, depression is manifested when a problem begins to affect fitness so much so that the costs associated with displaying depression are lower than the cost of not getting help.

Depression’s effectiveness as an adaptation is debatable. Between 3 to 5 percent of depressed individuals commit suicide, and up to 60 percent of those who commit suicide suffered from depression or other related mental illnesses. Suicide attempts themselves can be seen as signaling the seriousness of the depressive episode and usually occur after a protracted period of suicidal ideation (113). However, it is possible that depression led to successful outcomes in the ancient environment in which it evolved. Social conditions have changed drastically in a short period of time, and like the human craving for sugar, depression’s once-adaptive quality has taken a turn for the maladaptive. Additionally, the two discussed theories do not sufficiently address those bouts of depression that descend upon an individual for seemingly no reason.

The jury is still out on whether depression is indeed an adaptation stuck in the wrong time period (similar to diabetes) or whether it more resembles a feedback loop gone haywire (similar to chronic pain). While the analytical rumination hypothesis and the honest signaling hypothesis provide evidence for the former, the multi-faceted nature of depression makes it difficult to reach a consensus. Evolutionary psychology is an understandably murky field due to the difficulty in evidence-gathering. However, looking at perplexing mental phenomena through an evolutionary lens may help to uncover more effective therapeutic techniques.

Works Cited

Andrade, Laura, et al. “The epidemiology of major depressive episodes: results from the
International Consortium of Psychiatric Epidemiology (ICPE) surveys.” International Journal of Methods in Psychiatric Research. 12 (1):  3–21. ( 2003).


Andrews, Paul W., et al. “The functional design of depression’s influence on attention: A
preliminary test of alternative control-process mechanisms.” Evolutionary Psychology. 5(3): 584-604. (2007). http://www.epjournal.net/wp-content/uploads/ep05584604.pdf

Goodwin, G. M. “Neuropsychological and neuroimaging evidence for the involvement of the frontal lobes in depression.” Journal of Psychopharmacology, 11, 115 -122. (1997).

Hagen, Edward H. “The Bargaining Model of Depression.” Genetic and Cultural Evolution of
Cooperation. Ed. Peter Hammerstein. (2003).

Thomson, J. Anderson, et al. “The bright side of being blue: Depression as an adaptation for analyzing complex problems.” Psychological Review. 116(3): 620–654. (2009).


1 Comment

  1. What is the evolutionary advantage of depression?…

    Along with the rumination hypothesis, I find the honest signaling hypothesis of depression really interesting. It suggests that depression is an alternative to physical aggression as a form of social signaling that is meant to re-negotiate the terms of…

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