Maternal Investment and Postnatal Depression - An Evolutionary Approach

Postnatal depression is detrimental to maternal health and wellbeing, associated with poor developmental outcomes in children, and has prevalence estimates ranging from 13-60%; as such it is of significant public health concern and its origins are of interest from an evolutionary perspective. A growing movement within evolutionary research highlights the utility of evolutionary theory to elucidate the origins of health issues and indicate both novel approaches to treatment and prevention. A relatively longstanding, yet largely untested, existing evolutionary approach to postnatal depression proposes that it is a mechanism facilitating maternal investment decisions. More recently it has also been framed, somewhat complementarily, as the result of an evolutionary mismatch.

Using the responses to a retrospective survey study which collected the complete reproductive histories of women and was uniquely designed to capture their experiences of postnatal depression, the first data chapter of this thesis explores whether there is support for adaptationist hypotheses that postnatal depression exhibits good design as a mechanism guiding maternal reproductive trade-offs. The results, combined with critiques put forward here and by other authors, suggest an alternative approach to postnatal depression is warranted.

A limitation of both evolutionary and more traditional approaches to postnatal depression is that the commonly recognised risk factors for the condition fail to capture all the women who develop the condition. Recent developments in research into general depression, as opposed to postnatal depression, have highlighted the role of the immune system in symptom aetiology. This has led to a number of evolutionary researchers proposing that depression reflects an evolved inflammatory response to biological and social threat, with perceived social threat acting as an indicator of the likelihood of imminent biological threat. Inflammation then acts as the ultimate risk factor in the causal pathway to depression, and by extension postnatal depression, and suggests more attention needs to be paid to the social perceptions of women during pregnancy and early motherhood.

Data chapters 3-6 explore the social pressures surrounding women about motherhood, the role such pressures play in generating feelings of shame (an emotional marker of social threat causally linked to general depression development), and the ability of shame to predict postnatal depression. Particular attention is paid to pressures surrounding socially approved levels of maternal investment, namely in the form of bonding. Bonding is of interest due to the documented association between postnatal depression and poor bonding as well as the pressures placed on women in contemporary, developed populations, highlighted by sociologists and feminist scholars, as a result of the emphasis on the importance bonding for child development. The role of social isolation, another form of social threat linked to general depression, in postnatal depression risk is also assessed. In so doing, a new model for maternal emotional investments is developed based on embodied capital theory and the results of two further data sets are presented - the first is a longitudinal survey study tracking women across the perinatal period assessing their experience of social pressure, shame, and postnatal depression, and the second an experimental priming study designed to assess if social threat can be primed using popular and social media relating to mothering. Results derived from these studies are supportive of the perception of social threat being a largely unrecognised risk factor in postnatal depression and the thesis concludes with a discussion of the public health implications which stem from this novel insight.