Male infants, risk, and postnatal depression: Evidence supporting the Trivers-Willard hypothesis in a contemporary low-fertility context

Maternal care is obligate in mammals to ensure offspring survivability; however levels of investment are flexible dependent on resource availability. Parental investment theory predicts a mother should cease investing in an individual offspring when the benefits of the investment to her inclusive fitness are outweighed by the costs. The Trivers-Willard hypothesis predicts that a son of high quality will out-reproduce a daughter of comparable quality, while a daughter will have higher reproductive success than a son if both are of low quality. Postnatal depression is suggested by some evolutionary theorists to reflect active withdrawal of maternal investment in humans under conditions where continued investment is too costly. If this is the case then it should be expected to 1) conform to Trivers-Willard predictions and be both more common in association with sons than daughters, and 2) be more common in circumstances in which investment in sons is more costly. We test these hypotheses using data collected from a survey of women's complete reproductive histories which utilised retrospective measures of postnatal depression. Multilevel modelling showed that women were more likely to suffer postnatal depression after the births of sons than after the births of daughters, when controlling for other postnatal depression risk factors. Having sons was also found to elevate postnatal depression risk when mothers experienced low social support or complicated births. This is a novel and important finding of which healthcare professionals should be made aware.