Darwinian models of depression: a review of evolutionary accounts of mood and mood disorders.

Over the last ten years, there has been increased interest in the evolutionary origins of depressive phenomena. The current article provides a review of the major schools of thought that have emerged in this area. First, we consider important Darwinian explanations of depressed mood, including an integrative social risk hypothesis recently proposed by the authors. According to the social risk hypothesis, depression represents an adaptive response to the perceived threat of exclusion from important social relationships that, over the course of evolution, have been critical to maintaining an individual's fitness prospects. We argue, moreover, that in the ancestral environment, depression minimized the likelihood of exclusion by inducing: (i) cognitive hypersensitivity to indicators of social risk/threat; (ii) signaling behaviours that reduce social threat and elicit social support; and (iii) a generalized reduction in an individual's propensity to engage in risky, appetitive behaviours. Neurobiological support for this argument is also provided. Finally, we review three models that endeavour to explain the relationship between the adaptations that underlie depressed mood and clinically significant, depressed states, followed by a consideration of the merits of each.