The Mother–Offspring Conflict: The Association Between Maternal Sleep, Postpartum Depression, and Interbirth Interval Length

Abstract

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.

To What Extent Can Evolutionary Theories Explain Mental Illness?

“…Many psychological issues we consider 'disorders' today, such as the four mentioned above, are products of selection balance and trade-offs. For example, postpartum depression symptoms could be adaptive from the offspring's fitness' point of view by being associated with longer IBIs [11]. Agoraphobia can be understood as a mechanism to protect oneself from dangerous events in the past, which is used to be conducive to reproduction.…”

“…From the perspective of offspring, postpartum depression is positively correlated with the Interbirth Interval Length, meaning that the older one can benefit from postpartum depression [11]. Because the longer the birth interval age, the more conducive the concentration of resources to the offspring individuals.…”