A while ago I wrote about Andrews and Thomson’s adaptive rumination hypothesis (ARH) of depression, which holds that depression is an evolutionary adaption designed to help us solve difficult problems. I linked to two critiques (1, 2) of ARH by Jerry Coyne, who is clearly no fan of ARH. Coyne’s now taken his argument to the pages of Psychiatric Times, where he tears ARH to shreds for a third time. The main thrust of Coyne’s argument is that Andrews and Thomson employ a colloquial definition of adaptation (i.e., something that’s useful) rather than the more appropriate evolution definition:
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.
Andrews and Thomson don’t do this, or even try. And if they did try, they probably wouldn’t succeed, for everything we know about depression suggests that rather than enhancing fitness, it reduces it. The most obvious issue is suicide, a word that, curiously, does not appear in Andrews and Thomson’s text. Statistics show that those with major depression are 20 times more likely to kill themselves than are individuals in the general population. Evolutionarily speaking, this is a strong selective penalty. Depression also appears to reduce libido and may make one unattractive as a sexual partner. Andrews and Thomson point out depression’s “adverse effect on women’s fertility and the outcome of pregnancy.“ Other health problems are comorbid with depression, although it’s not clear whether depression is the cause or consequence of these problems. Finally, studies show that depressed mothers provide poorer care of their children.
As Coyne notes, this is a problem not only for ARH, but also for a number of other evolutionary psychological accounts of depression–essentially, all those theories that posit that the depressive state itself is adaptive (as opposed to balancing selection/heterozygote advantage models which allow for the possibility that some genes that contribute to depression may be selected for under the right circumstances, without implying that depression itself is advantageous).