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Offspring: Human Fertility Behavior in Biodemographic Perspective
we are a long way from carrying out such a program but not so far from understanding how some modern behavior may be rooted in the early exigencies of lineage survival. Such an evolutionary perspective is introduced in Chapter 7 by Hillard Kaplan and Jane Lancaster. Underlying their approach is the idea that natural selection has less to do with specific sequences of behavior and more to do with “norms of reaction,” systems for adjustable responses to environmental stimuli. The celebrated diversity of human mating arrangements and reproductive norms and practices may reflect common evolutionary principles working themselves out in highly diverse ecological settings.
Kaplan and Lancaster’s chapter is representative of a line of thought in which a kind of economic calculus takes its place alongside the traditional reproductive calculus in the reckoning of long-term fitness in the face of natural selection. This economic calculus involves constraints imposed by feasible age-specific production and consumption, returns to skill, social support, and resource transfers across generations.
In Kaplan and Lancaster’s account, the modern concept of a quality-quantity trade-off in the demand for children has an analog for hominid foragers. Compared to chimpanzees, hominids came to concentrate on an ecological food niche, including hunted prey and extracted nutrients, which demanded and rewarded skill and learning. Prolonged juvenile training and dependence, protracted parental investment, larger brains, and longer life spans are seen as coevolving, driven by returns to investment in “embodied capital.” In this picture, evolution would have been equipping humans not so much with an instinct toward maximizing total fertility as with instincts for adjusting familial resource transfers in response to the available lifetime returns to such investments.
Elements of this picture are subject to lively debate. An alternative reading of the paleoanthropological data would see the distinctive pacing of human life histories, with extended gestation, juvenile dependence, and training, as tending to precede rather than accompany the evolution of larger and more sophisticated brains. Views differ on how much weight to give to direct provisioning and how much to concomitant gains in status that could enhance mating opportunities. The area is one of active research.
Having outlined their overall perspective, Kaplan and Lancaster proceed in Chapter 7 with a whirlwind tour through human history, from foragers on to horticulturalists, pastoralists, agricultural civilizations, and industrialized societies. They point out along the way relationships between ecological settings and observed outcomes for male-female complementarity in provisioning and parenting, mating and marital systems, and age-specific economic returns. The modern examples are drawn especially from a U.S. perspective. Like the chapters that follow, Chapter 7 provides an entrée to an extensive corpus of existing scientific work.