Structured Abstract Background and objectives As the mother-offspring relationship is central to human reproduction, postpartum depression symptoms are difficult to explain in evolutionary terms. We proposed that postpartum depression might arise as a result of evolutionary mother-offspring conflict over maternal investment, and investigated the association between postpartum depression symptoms, infant night waking, maternal sleep disturbance and breastfeeding frequency. Methodology We conducted a cross-sectional analysis using survey responses at six months postpartum from 1598 Finnish mothers. We hypothesized that infant night waking at six months postpartum would be associated with postpartum depression symptoms, and that this association would be mediated by maternal sleep disturbance and a higher breastfeeding frequency. Results Infant night waking was moderately associated with postpartum depression symptoms, and this association was mediated by maternal sleep disturbance (R2 = .09). Contrary to our prediction, we found that increased breastfeeding was associated with less postpartum depression symptoms. Conclusions and implications We conclude that postpartum depression symptoms might partly be the result of increased maternal fatigue stemming from high offspring demands on maternal investment, but that this is not due to the metabolic strain from increased breastfeeding. Studying postpartum depression from the mother-offspring conflict perspective can potentially improve our understanding of the involved behavioral processes of both mother and offspring, and allow interventions designed to benefit the well-being of both parties.

Background and objectives: As the mother-offspring relationship is central to human reproduction, the high incidence of postpartum depression symptoms is difficult to explain in evolutionary terms. We proposed that postpartum depression might be the adverse result of evolutionary mother-offspring conflict over maternal investment, and investigated the association between postpartum depression symptoms, infant night waking, maternal sleep disturbance and breastfeeding frequency. Methodology: We conducted a cross-sectional analysis using survey responses at six months postpartum from 1598 Finnish mothers. We hypothesized that infant night waking at six months postpartum would be associated with postpartum depression symptoms, and that this association would be mediated by maternal sleep disturbance and a higher breastfeeding frequency.Results: Infant night waking was associated with postpartum depression symptoms, and this association was mediated by maternal sleep disturbance. Contrary to expectations, we found that the increased breastfeeding was associated with less postpartum depression symptoms. Conclusions and implications: We conclude that postpartum depression symptoms might be the result of increased maternal fatigue stemming from high offspring demands on maternal investment, but that this is not due to the metabolic strain from increased breastfeeding. Studying postpartum depression from the mother-offspring conflict perspective improves our understanding of the involved behavioral processes of both mother and offspring, and thus allows interventions designed to improve the well-being of both parties.

To test the hypothesis that infant night waking is an adaptation to increase interbirth intervals (i.e., the time between a mother’s consecutive births) by exhausting the mother, we investigated whether maternal sleep disturbance is associated with longer interbirth intervals. We also explored whether postpartum depression symptoms mediated the association between maternal sleep disturbance and interbirth intervals. We used retrospective self-reports from 729 mothers living in Finland. We conducted structural regressions separately for the mother’s two first children at two different age intervals (0–1 and 1–3 years). Infant night waking was associated with maternal sleep disturbance (β = .78–.84) and maternal sleep disturbance was associated with postpartum depression symptoms (β = .69–.81). Postpartum depression symptoms were also associated with longer interbirth intervals for the first child (β = .23–.28). This result supports the notion that postpartum depression in and of itself could be viewed as adaptive from the offspring’s fitness’ point of view, and not just as an unintentional byproduct of the mother’s sleep disturbance. Contrary to our prediction, maternal sleep disturbance was, however, associated with shorter interbirth intervals for the first child (β = -.22– -.30). We discuss the potential role of social support as an explanation for this unexpected result.

To test the hypothesis that infant night waking is an adaptation to increase interbirth intervals (IBIs) (i.e., the time between a mother’s consecutive births) by exhausting the mother, we made an initial attempt at investigating whether maternal sleep disturbance is associated with longer IBIs. We also explored whether postpartum depression symptoms mediated the association between maternal sleep disturbance and IBI length. We used retrospective self-reports from 729 mothers living in Finland. We conducted structural regressions separately for the mother’s two first children at two different age intervals (0–1 and 1–3 years). Infant night waking was associated with maternal sleep disturbance (β  =  .78–.84) and maternal sleep disturbance was associated with postpartum depression symptoms (β  =  .69–.81). Postpartum depression symptoms were also associated with longer IBIs for the first child (β  =  .23–.28). This result supports the notion that postpartum depression in and of itself could be viewed as adaptive for the offspring’s fitness, and not just as an unintentional byproduct of the mother’s sleep disturbance. Contrary to our prediction, maternal sleep disturbance was, however, associated with shorter IBIs for the first child (β  =  −.22 to −.30) when including postpartum depression symptoms in the model. We discuss the potential role of social support as an explanation for this unexpected result.