foods. The logic underlying this proposal is that for humans, effective adult foraging requires an extended developmental period during which production at young ages is sacrificed for increased productivity later in life. The returns to investment in development depend positively on adult survival rates, favoring increased investment in mortality reduction. An extended postreproductive, yet productive, period supports both earlier onset of reproduction by next-generation individuals and the ability to provision multiple dependent young at different stages of development.

Two distinct possibilities regarding the evolution of the postreproductive period were considered. One is that menopause evolved to facilitate postreproductive investment in offspring. The other is that reproductive senescence evolved due to the costs of maintaining viable oocytes and that increased longevity evolved, in spite of menopause, to support the reproduction of descendants.

This theory was developed as part of a more general theory of the evolution of life histories. Two major tradeoffs were considered. First, resources can be invested in either current or future reproductive effort. Investments in future reproductive effort include both those that enhance survival and increase future income (in a general sense). Age-specific allocations that maximize the lifetime allocation to reproductive effort will be favored by natural selection. Second, there is a tradeoff between quantity and quality of offspring. The specific model of human life-history evolution proposes that compared to other primates, traditional human ecology favored higher levels of investment in both future reproduction and quality of offspring.

It is useful to think of short- and long-term responses to various environments and, hence, various optimal allocation regimes. Natural selection can favor the evolution of physiological and psychological mechanisms that facilitate short-term adjustments to environmental variation. The degree of phenotypic plasticity that evolves will represent a compromise between the costs and benefits of flexible responses and also reflect the range of environmental variation experienced by the organism. Humans clearly demonstrate a high degree of adaptive flexibility, mediated through both physiology and behavior. Although the mechanisms underlying our response system evolved in the context of a hunting and gathering way of life, this evolved flexibility is apparent in our recent history as well. Changes in investments in income-related capital, mortality reduction, and maintenance associated with the demographic transition may reflect increased returns to those investments, stimulated by the increasing importance of skills-based competitive labor markets. Similarly, within developed countries, those that have more to gain from investments in education also invest more in longevity and health.

Long-term adjustments occur when one short-term response system is competitively more effective than another response system. In general, this would occur when environments change sufficiently so that the ancestral response system produces unfavorable outcomes. We cannot expect natural selection to have



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