The life-history trade-off between fertility and child survival

Evolutionary models of human reproduction argue that variation in fertility can be understood as the localoptimization of a life-history trade-off between offspring quantity and ‘quality’. Child survival is a funda-mental dimension of quality in these models as early-life mortality represents a crucial selective bottleneckin human evolution. This perspective is well-rehearsed, but current literature presents mixed evidence fora trade-off between fertility and child survival, and little empirical ground to evaluate how socioecologicaland individual characteristics influence the benefits of fertility limitation. By compiling demographicsurvey data, we demonstrate robust negative relationships between fertility and child survival across27 sub-Saharan African countries. Our analyses suggest this relationship is primarily accounted for by off-spring competition for parental investment, rather than by reverse causal mechanisms. We also findthat the trade-off increases in relative magnitude as national mortality declines and maternal somatic(height) and extrasomatic (education) capital increase. This supports the idea that socioeconomic devel-opment, and associated reductions in extrinsic child mortality, favour reduced fertility by increasing therelative returns to parental investment. Observed fertility, however, falls considerably short of predictedoptima for maximizing total offspring survivorship, strongly suggesting that additional unmeasuredcosts of reproduction ultimately constrain the evolution of human family size.