Postnatal depression – an evolutionary overview

Postnatal depression (PND) presents a puzzling phenomenon to evolutionary anthropologists as it is highly prevalent and yet detrimental to child development and maternal health. Adaptationist explanations use parental investment theory to propose that it is an adaptive signal to a mother that she is experiencing a cost to her fitness by investing in a particular offspring and should therefore reduce/eliminate investment, while displays of distress elicit support from kin. Mismatch hypotheses on the other hand contend PND’s aetiology lies in ‘modern’ parenting styles and developed environments. A third line of explanation views PND as an immune response triggered by external stressors, tying it in with recent evolutionary explanations of depression more generally which see it as akin to sickness behaviour in mammals and thus a maintenance strategy in response to threat. Until now very little direct evidence has been brought to bear on the fitness consequences of PND which may enable the relative merits of these explanatory frameworks to be weighed. Here we present data on the effects of PND from two populations that vary in socio-economic setting; a Western sample and the Tsimane forager-horticulturalists from Bolivia. In the Western sample, furthering recently published work, we examine two fitness relevant measures – fertility and relationship quality – over two generations. In the Tsimane we examine the relationship between PND and fertility. Drawing comparisons we assess whether patterns documented in the Western sample are unique to Westernised contracepting populations, discuss the implications for the various evolutionary explanations, and suggest areas for future research.