Millions of people around the world suffer from depression, the most common mental disorder of all. Since depression appears to be largely genetic, several long-standing questions continue to bedevil researchers. Have the genes for clinical unipolar depression undergone selective evolution–or is depression a random product of mutation, evolutionary drift, or other non-selective forces?

The symptoms of depression are found in every culture and time period, from the ancient Greeks to modern New Yorkers, from the !Kung of southern Africa to ranchers in the American West. Why is depression so much more common than any other major mental illness? Clearly, it is a malfunction, a maladaptation — or is it?

What if depression sometimes turns out to be a useful adaptation, rather than a malfunction?  When people are depressed, they spend more time thinking, and less time engaged in physical activity. Paul W. Andrews, a researcher with the Virginia Institute for Psychiatric and Behavioral Genetics at Virginia Commonwealth University, believes that “depression is an evolved emotional response to complex problems, and its function is to promote changes in body systems that promote analysis of those problems.”

In a recent paper for Psychological Review — “The bright side of being blue” — Paul Andrews and Anderson Thomson propose the “analytical rumination hypothesis” to explain the widespread occurrence of depression. As an evolved response to solving complex problems, depression’s function is to “minimize disruption and sustain analysis of those problems by… reducing the desire to engage in distracting activities (anhedonia), and producing psychomotor changes that reduce exposure to distracting stimuli.”

Writing for ScientificAmerican.com, the two researchers argued that depression involves a specific, highly analytical thinking style.  Faced with a difficult math problem, “feeling depressed is often a useful response that may help you analyze and solve it,” the authors write. They claim to have found evidence that “people who get more depressed while they are working on complex problems in an intelligence test tend to score higher on the test.”

The authors point to various lab experiments indicating that depressed people may be better at solving complex social dilemmas as well, because they give more scrutiny to the costs and benefits of various options. What may look like indecision, or an inability to act decisively, may be artifacts of a particular problem-solving style; a cognitive technique that requires a minimum of outside distractions.

Viewed in this light, certain symptoms of depression — social isolation, an inability to derive pleasure from pleasurable acts (like sex), and a loss of appetite — combine to maximize the brain’s ability to focus and process information. This combination of cognitive and psychomotor effects might have the adaptive function of putting the brain in the perfect gear for certain kinds of complex problem solving.

The theory is far from watertight. Yes, depressives are capable of monumental feats of rumination and contemplation. But is obsessive brooding always a fruitful technique? As anyone who has dealt with a depressed person knows, the conclusions reached by all this high-level problem solving are often completely erroneous.  In many cases, depressives seem to be even more prone to fallacious thinking than non-depressed problem solvers.

The debate over the usefulness of depression shows no signs of early or easy resolution. But the search for the adaptive significance of mental and emotional traits always carries with it the possibility of major insights into evolutionary biology.

References

Andrews, P., Thomson Jr., J. (2009). Depression’s Evolutionary Roots. Mind Matters, August 25, scientificamerican.com.

Andrews, P., & Thomson, J. (2009). The bright side of being blue: Depression as an adaptation for analyzing complex problems. Psychological Review, 116 (3), 620-654 DOI: 10.1037/a0016242

Watson, P. (2002). Toward a revised evolutionary adaptationist analysis of depression: the social navigation hypothesis Journal of Affective Disorders, 72 (1), 1-14 DOI: 10.1016/S0165-0327(01)00459-1

Hertel, Jochen Neuhof, Thomas Theue, G. (2000). Mood effects on cooperation in small groups: Does positive mood simply lead to more cooperation? Cognition & Emotion, 14 (4), 441-472 DOI: 10.1080/026999300402754

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