Evolutionary origins of depression: a review and reformulation.

There has been a recent surge of interest in the evolutionary basis of depression. One approach argues that the affective mechanisms that are dysregulated in depression are adaptations, whilst a second approach argues that depression itself is an adaptation. The evidence relating to whether depression could itself be an adaptation is reviewed. Adaptations generally have four hallmarks; they lack heritable variation, show evidence of good design, are evoked by appropriate triggers, and fitness is reduced where they are absent. Depression shows none of these hallmarks. It is characterized by heritability, recurrence, cognitive impairment, and poor social outcome. In an alternative evolutionary formulation, I argue that evolution has produced a continuous population distribution of affective reactivity that is subject to stabilizing selection. Individuals vulnerable to depression are at the upper end of this distribution. This conceptualization, in which depression itself is not selected for, is compatible with the known clinical and epidemiological facts.