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

Abstract

Evolutionary models of human reproduction argue that variation in fertility can be understood as the local optimization of a life-history trade-off between offspring quantity and ‘quality’. Child survival is a fundamental dimension of quality in these models as early-life mortality represents a crucial selective bottleneck in human evolution. This perspective is well-rehearsed, but current literature presents mixed evidence for a trade-off between fertility and child survival, and little empirical ground to evaluate how socioecological and individual characteristics influence the benefits of fertility limitation. By compiling demographic survey data, we demonstrate robust negative relationships between fertility and child survival across 27 sub-Saharan African countries. Our analyses suggest this relationship is primarily accounted for by offspring competition for parental investment, rather than by reverse causal mechanisms. We also find that 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 development, and associated reductions in extrinsic child mortality, favour reduced fertility by increasing the relative returns to parental investment. Observed fertility, however, falls considerably short of predicted optima for maximizing total offspring survivorship, strongly suggesting that additional unmeasured costs of reproduction ultimately constrain the evolution of human family size.

What do men want? Re-examining whether men benefit from higher fertility than is optimal for women

“…This inconsistency means the same empirical association of kin presence with higher fertility is often interpreted as evidence of altruism, or of coercion, depending on whether the family member is a woman's mother [57,59] or motherin-law [11,60], respectively. In fact, in most places where anthropologists have tried to measure the relationship between fertility and fitness, including in high fertility contexts with limited effective contraception, these are positively and monotonically related [61][62][63][64]. This does not necessarily mean that one or both arguments must be incorrect: a woman's costs to shorter IBIs can be lowered by supportive social relations, and she can be coerced into a shorter (or longer) IBI by a mate unsure about his future paternity.…”

Faster reproductive rates trade off against offspring growth in wild chimpanzees

“…Many studies find the predicted negative relationship between reproductive rates and offspring growth or health (14)(15)(16)(17)(18) or demonstrate fitness maximization at intermediate levels of fertility (19)(20)(21). For example, when a water tap was installed to reduce women's workload in a rural Ethiopian community, birth rates increased, but child growth rates decreased, and more children were clinically stunted (22).…”

The Genetic and Environmental Structure of Reproduction-Related Variables: The Case of Fertility and Breastfeeding

“…This relationship would be modulated by physiological processes (Kaplan & Lancaster, 2003) as well as by socioeconomic characteristics that influence the relative returns to parental investment (Lawson et al, 2012). However, there is a lack of information about how these variables relate to each other, and a scarcity of studies that could present evidence regarding the shape of such an association.…”