Parental investment and the optimization of human family size

Abstract

Human reproductive behaviour is marked by exceptional variation at the population and individual level. Human behavioural ecologists propose adaptive hypotheses to explain this variation as shifting phenotypic optima in relation to local socioecological niches. Here we review evidence that variation in fertility (offspring number), in both traditional and modern industrialized populations, represents optimization of the life-history trade-off between reproductive rate and parental investment. While a reliance on correlational methods suggests the true costs of sibling resource competition are often poorly estimated, a range of anthropological and demographic studies confirm that parents balance family size against offspring success. Evidence of optimization is less forthcoming. Declines in fertility associated with modernization are particularly difficult to reconcile with adaptive models, because fertility limitation fails to enhance offspring reproductive success. Yet, considering alternative measures, we show that modern low fertility confers many advantages on offspring, which are probably transmitted to future generations. Evidence from populations that have undergone or initiated demographic transition indicate that these rewards to fertility limitation fall selectively on relatively wealthy individuals. The adaptive significance of modern reproductive behaviour remains difficult to evaluate, but may be best understood in response to rising investment costs of rearing socially and economically competitive offspring.

Land inheritance establishes sibling competition for marriage and reproduction in rural Ethiopia

“…Among the first to adopt the use of modern contraception are those farmers who have inherited the smallest parcels of land (β − 0.61 ± 0.28, P = 0.032; Table 2). These findings contribute to growing evidence that competition for inelastic heritable resources, such as land, cattle, and titles, represents an important determinant of the nature of sibling relationships and also parental reproductive decision making (6,19).…”

“…Recent anthropological studies in pretransitional societies, however, have found mixed evidence of adult sibling resource competition (19). In some instances positive effects of siblings have been demonstrated, suggesting that the costs to resource division within the family have the potential to be offset by beneficial cooperative activities between siblings (see reviews of human cooperative breeding strategies in refs.…”

“…Current studies also differ in their adjustment for relevant socioeconomic variation (and associated phenotypic correlations) between families, which may distort the consequences of sibling competition. For example, sibling rivalry may be more evident in agricultural and pastoral populations partly because it is easier to measure and adjust statistically for differences in resource access between families (19).…”

Evolutionary accounts of human behavioural diversity

“…Typically, such cultural inheritance patterns would be interpreted as simply one mechanism through which our species adapts its behaviour to the local ecology, but this human behavioural ecology perspective also does not necessarily preclude the notion that such cultural inheritance patterns could potentially operate independently of the influences of ecological variables. For example, human behavioural ecologists have successfully applied optimality modelling to recent life-history changes, such as low fertility in postdemographic populations [75], despite the suggestion that post-industrial cultural change has occurred too quickly to avoid a 'mismatch' or 'adaptive lag' between the environment and our evolved behavioural strategies. Incorporating socially learned transmission of information is therefore not inevitably incongruent with the human behavioural ecology perspective [76,77].…”

What if fertility decline is not permanent? The need for an evolutionarily informed approach to understanding low fertility

“…Likewise, in many cases, the evolutionary and the mainstream approaches are quite similar. For example, the quantity-quality offspring trade-off is quite similar in certain areas of demography and evolutionary anthropology [16,[63][64][65]. The role of cultural norms, and how they spread, is an emerging area of study in both social demography and evolutionary demography [34,45,66], which makes it an area of potential collaboration between fields.…”