Effects of female reproductive competition on birth rate and reproductive scheduling in a historical human population

Costly reproductive competition among females is predicted to lead to strategies that reduce these costs, such as reproductive schedules. Simultaneous births of co-resident women in human families can reduce their infant survival, but whether such competition also affects overall birth rates and whether females time their pregnancies to avoid simultaneous births remain unknown, despite being key questions for understanding how intra-female competition affects reproductive strategies. Here, we used detailed parish registers to study female reproductive competition in historical Finnish joint –families, where brothers stayed on their natal farms and sisters married out, and consequently unrelated daughters-in-law often co-resided and competed for household resources. We quantified the time-varying effects of having reproductive-aged competitor(s) on a woman’s interval from marriage to first childbirth, on age-specific fertility, and on birth scheduling. Contrary to our hypothesis, the pre...