This paper has two interrelated goals. The first is to offer a general theory of fertility and parental investment across a broad spectrum of human societies. The second is t o provide a perspective that unifies traditionally separate domains of anthropology. The basic foundation for the analysis is life history theory and evolutionary biological models of optimal fertility regulation. This tradition is combined with human capital theory in economics to produce a more general theory of investments in embodied capital within and between generations. This synthesis results in a series of optimality models to examine the decision processes underlying fertility and parental investment upon which natural selection is expected to act. Those models are then applied to the hunting and gathering lifeway. This analysis focuses both on problems that all hunting and gathering peoples face and on the production of variable responses in relation to variable ecologies. Next, this consideration of optimal parental investment and fertility behavior in hunter-gatherers is united with existing models of the proximate determinants of human fertility. The analysis of proximate mechanisms is based on the idea that natural selection acts on the final phenotypic outcome of a coordinated system of physiological, psychological and cultural processes. The important conditions affecting parental investment and fertility in modern socioeconomic contexts are then discussed. An explanation of modern fertility and parental investment behavior in terms of the interaction of those conditions with the physiological and psychological mechanisms that evolved during our hunting and gathering history is proposed. The proposal is that skills-based competitive labor markets increase the value of parental investment in children and motivate better-educated, higher income parents to invest more per child than their less-educated, lower-earning counterparts. It is also suggested that the deviation from fitness maximization associated with low modern fertility is due to excess expenditures on both parental and offspring consumption, indicating that our evolved psychology is responding to cues in the modern environment that are not directly related to the fitness impacts of consumption. 0 1996 Wiley-Liss, Inc.
'Becker (1975) and Ben-Porath (1967) obtain similar results in the analysis of investments in human capital.