Local ecology influences reproductive timing in Northern Ireland independently of individual wealth

Abstract

Evolutionary models of human life-history predict that ecological characteristics drive variability in reproductive timing by altering anticipated returns to inclusive fitness. Local extrinsic mortality rate (EMR), crime (CR), and female-biased sex ratios have all been predicted to accelerate reproduction. However, previous research has failed to isolate the impact of these ecological characteristics from individual factors, such as wealth. Here, we utilize a unique longitudinal dataset from Northern Ireland (570 electoral wards; 62339 individuals) that enables us to address this issue and to apply a novel measure of extrinsic mortality based on a definition from public health. We demonstrate that high ward-level EMR, CR, and female-biased sex ratios have additive positive impact on the risk of early motherhood and that CR and EMR predict early fatherhood. These effects remained significant after adjustment for potentially confounding factors but were greatly attenuated when individual-level socioeconomic characteristics were adjusted for. Our findings suggest that young individuals in this population are sensitive to several ecological cues, including local crime and adult sex ratio, which speed up first birth over and above the strong effects of individual wealth.