Abstract: Postpartum depression poses an evolutionary puzzle: it is extremely common, yet significantly reduces the reproductive fitness of both mothers and children. Why has natural selection failed to remove this trait? I will consider the hypothesis that postpartum depression represents a “disease of modern civilization” – that is, a byproduct of the dramatic cultural changes to motherhood that have occurred over the last century. This perspective predicts that postpartum depression will be more common in contexts where breastfeeding, diet, exercise, sleep, & alloparenting patterns diverge most dramatically from those of our Pleistocene ancestors. I will present cross-cultural, epidemiological, and experimental studies showing that postpartum depression is associated with early weaning, insufficient vitamin D due to limited sun exposure, diets deficient in essential fatty acids, and isolation from kin support networks, all of which diverge significantly from lifestyles typical throughout most of human evolution.