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The Evolutionary Calculus of Depression

The Evolutionary Calculus of Depression

The discipline of evolutionary psychology views modern human behaviors as products of natural selection that acted on the psychological traits of our ancestors. A subdiscipline, evolutionary psychiatry, tries to find evolutionary explanations for mental disorders.

One of the most common subjects of evolutionary psychiatry is depression. Although debilitating, depression is also reasonably widespread. Estimates of its prevalence in Western nations range between 5% and 20%, and the disorder appears to depend at least partly on an individual’s genes. The relatively high frequency of an apparently maladaptive and partially genetic syndrome has led to speculation that it may really be “adaptive” in an evolutionary sense—that is, a liability to depression may have been installed in our genome by natural selection.

A recent version of this idea is the adaptive rumination hypothesis (ARH) of Andrews and Thomson,1 which posits that depression evolved as a way to solve difficult and complex problems, most of them involving social interactions. Instead of being a pathology, depression is seen as a useful complex of thoughts and behaviors that enable troubled people to withdraw from the world, deliberate intensively about their social problems, and devise solutions. Andrews and Thomson suggest this behavior evolved because it was adaptive in our ancestors, and may still be so.

The ARH has attracted a good deal of attention, much of it favorable. It was, for example, the subject of a recent article in The New York Times magazine.2 Debate about the ARH is not purely academic, for Andrews and Thomson see the idea as pointing to specific therapies, including problem-solving talk therapies and the deliberate withholding of medication. Since these suggestions stem from a specific evolutionary hypothesis, we should carefully examine that hypothesis.

I have previously discussed a number of troubling problems with the ARH.3 Andrews and Thomson deliberately conflate clinical depression and simple sadness, assuming that these are simply positions on a continuous psychological gradient. They give no evidence that depression is caused by, rather than the cause of, difficult social problems, and they fail to show that depression actually helps people solve those problems. And many of the “experiments” supporting the ARH are unrealistic, bordering on silly. One such study mimicked the effect of depression on problem solving by having people engage in mock currency trading while listening to sad music.

Rather than repeat my critique, I want to discuss how evolutionary biologists identify features as “adaptations” and relate this to evolutionary explanations of mental disorders such as the ARH. I will show that depression does not meet the minimal requirements for qualifying as a biological adaptation, and that even if it did, the evolutionary explanation of the ARH—and of other “adaptive” theories for depression—is scientifically unsound.

The ARH is unsatisfactory for three reasons:

Depression is not an adaptation in the evolutionary sense. Andrews and Thomson consider depression an “adaptation” because it supposedly helps the sufferer solve problems. But an evolutionary adaptation is more than something that is merely useful. Biologists consider a trait adaptive only if that behavior, and the genes producing it, enhance an individual’s fitness—the average lifetime output of offspring. It is this genetic advantage, and the evolutionary changes in behavior it promotes, that is the essence of adaptation by natural selection. To demonstrate that depression is an evolved adaptation, then, we must show that it enhances reproduction.

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