Steven Gangestad, in Chapter 8, describes how various competing interests of the male and female members of a couple exist alongside their common interest in offspring. He intentionally divides the topic with Kaplan and Lancaster, focusing on conflicting stakes in parenting while they focus on complementarities. Biological processes in which male and female members of a species successively adapt to each other’s own gene-promoting stratagems, in a generation-by-generation dance of action and reaction, is called “sexually antagonistic coevolution.” Gangestad reports on experiments with Drosophila, dung flies, and finches showing such coevolution taking place. He reviews the theoretical implications for humans and describes observations of human behavior under controlled conditions that exemplify the predicted patterns. Among the surprises featured by Gangestad is an increase in a suite of “mate retention tactics” on the part of husbands specifically during the ovulatory phase of their wives’ cycles. During the same phase, wives report fantasizing more often about other males. The biological studies provide a context for consideration of contemporary social phenomena in conflicts over parenting effort and marital instability.
Ben Campbell shifts the focus in Chapter 9 to adolescence, specifically reproductive maturation in boys. The topic goes beyond physical sexual maturation to include the endocrinological regulation of emotional and social development. Campbell advances a hypothesis of his own about the possible roles for two adrenal hormones acting progressively over an extended period from just before puberty into early adulthood. Campbell reflects on the evolutionary trade-offs implicated in this part of the human developmental program, on the gap in years between puberty and procreation in many human societies, and on the reversed order of peak growth spurt and menarche for girls compared to puberty and peak growth for boys. He considers refinements of the basic idea that timing of sexual maturation balances the benefits of earlier reproductive opportunities against the costs of risky male-male competition. The availability of biological indicators from populations with differing levels of nutritional advantage and differing norms governing risk taking, competition, violence, affiliation, and mating hold out the promise of testing hypotheses in this area in the coming years.
In Chapter 10, Carol Worthman draws on three approaches—life history theory, reproductive ecology, and developmental psychobiology—and discusses their strengths and limitations and what is to be gained by bringing them together. It is a wide-ranging chapter, taking up a variety of aspects of fertility and family formation not fully treated elsewhere in the volume. Alongside genetic transmission, Worthman stresses the importance of biocultural inheritance. She features the costs of sociality in evolutionary settings, and their implications for established patterns of behavior. She