When not to have another baby: An evolutionary approach to low fertility

Abstract

AbstractBACKGROUNDIn this paper, I explain the theoretical foundations of fertility from the perspective of evolutionary demography and evolutionary anthropology.OBJECTIVEMy goal in this paper is to provide examples of how evolutionary demography is generating and testing new hypotheses about human fertility and parental behaviour.METHODSTo illustrate the paradigm of low fertility, I present several evolutionary explanations for reduced fertility, or no fertility at all. The explanations I cite are drawn from studies on child maltreatment, homosexual preference, post-demographic transition low fertility, and late-life low fertility (menopause), as these are phenomena that appear to challenge evolutionary approaches.CONCLUSIONSI find that the sophisticated tools of behavioural ecology and evolutionary anthropology and demography can do more than simply explain high fertility, and are currently being used to generate and test new hypotheses about fertility, including hypotheses that address the issue of low fertility.1. An introduction to evolutionary demographyEvolutionary demography (or the application of evolutionary theory to human demographic behaviour) rests on the assumption that all organisms are designed by natural selection. As evolutionary theory is an overarching theory that explains how all living things look, develop, and behave, it is not surprising that it is relevant to understanding human fertility. Natural selection is assumed to act on physiological and decision-making processes in such a way that these processes maximise inclusive fitness in the environment in which they evolved. Inclusive fitness is a measure of the number of relatives an individual produces, weighted according to how closely related they are to the individual by direct descent (Hamilton 1964).In this paper, I outline some recent developments in the application of evolutionary theory to human demography. Rather than reviewing the entire field, which is very large, I will illustrate this approach by tackling what might be considered the most challenging question raised in the discipline: namely, what is the evolutionary basis for the failure of some people to reproduce? A simplistic analysis might suggest that humans (and indeed any species) should, subject to constraints, be evolved try to reproduce at the maximum possible rate. I will examine how evolutionary demographers and biologists have explained three contexts in which humans either do not reproduce, or do so far less than is theoretically possible: homosexuality, menopause, and the demographic transition. I will conclude by responding to some of the theoretical issues the other contributors to this volume are also addressing.Biologists generally define evolutionary demography as the application of life history theory to population processes. Having worked in both the biological and the social sciences, I often switch between the two different vocabularies of the social and biological sciences. One difference between these scientific disciplines is that the phenomenon that demographers call fertility (the number of births, which is the definition I use), biologists call fecundity. The two fields are similar in many respects. Life history theory is a well-developed sub-discipline of evolutionary ecology which is explicitly concerned with the timing of life history events (growth, reproduction, maintenance, and death) under natural selection. The timing and the scheduling of births in order to maximise fitness (in the Darwinian sense) can in theory be calculated for any given environment and any given set of environmental constraints. In examining the timing of births, researchers often try to understand both the behavioural and the physiological determinants of fertility (e.g., menopause). Parental investment is viewed as a life history trait that is determined in part by considerations regarding the quantity versus the quality of offspring. …