conditions. However, substantial changes in the environment may select for a new norm of reaction, and this will occur over the longer run because it requires change in the distribution of genotypes in the population.

The most basic features of the psychological and physiological mechanisms underlying our set of responses to environmental variation evolved in the context of a hunting and gathering ecology. Given that environmental dangers, disease threats, food supply, and the importance of skill in food acquisition are likely to have varied across hunter-gatherer ecologies, we can expect that optimal life-history allocations would have varied as well. For example, food-intake rates undoubtedly varied through time, across habitats, and as a function of population density. The ability to alter allocations to survival, maintenance, reproductive effort, fertility, and parental investment in response to changing net energy-intake rates must have been under selection.

Part of the response system is under physiological control. The probability of having a fecund menstrual cycle varies positively with seasonal variation in net food-intake rates in food-limited populations (Ellison, 1990, 1995; Ellison et al., 1989; Huffman et al., 1978; Hurtado and Hill, 1990; Leslie and Fry, 1989; Lunn et al., 1984). In children, disease rates decrease and growth rates increase with increased food and protein intake, indicating a distribution of the extra food to several functions (for review, see Hill and Hurtado, 1996: chapter 10 and table 10-1). Part of the response system is under behavioral control. Both parents and children exert control over the introduction of weaning foods. Age-specific exposure to environmental risks of accident, intraspecific aggression, and predation depends, in part, on activity regimes. Because the behavioral and physiological responses interact in determining the final outcome (e.g., rates of breast feeding and food intake interact in determining probability of an ovulatory menstrual cycle), it is likely that selection would have produced a coordinated physiological/psychological response system that yields adaptive life-history adjustment in relation to changing conditions characteristic of hunter-gatherer ecologies.

Most people today live under very different conditions. What kinds of responses do we expect in relation to modern environments and to variability within modern environments? Very little is known about the answer to this question. It is perhaps the most fundamental question facing the social, behavioral, and medical sciences today. One working assumption is that environmental variation in modern societies qualitatively similar to variation in hunter-gatherer societies should produce similar responses to what would be expected under traditional conditions. The response to the changing importance of skill and education in the last century may be one such example.

The central prediction of the life-history model presented above is that payoffs to investment in income-related capital interact positively with payoffs to investment in survival in determining allocations to reproductive effort and embodied capital. One theory of the demographic transition (the dramatic reduction in fertility that occurred in Europe and America about 100 years ago) is that it is

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