The Better Angels of Our Nature: Why Vio - By Steven Pinker Page 0,255

is that many infanticidal tribes and civilizations were not environmentally stressed. A more serious problem is that it has the fatal flaw of all naïve good-of-the-group theories, namely that the mechanism it proposes is self-undermining. Any family that cheated on the policy and kept its daughters alive would take over the population, stocking it with their grandchildren while the excess bachelor sons of their altruistic neighbors died without issue. The lineages that were inclined to kill their newborn daughters would have died out long ago, and the persistence of female infanticide in any society would be a mystery.

Can evolutionary psychology explain the gender bias? Critics of that approach say that it is merely an exercise in creativity, since one can always come up with an ingenious evolutionary explanation for any phenomenon. But that is an illusion, arising from the fact that so many ingenious evolutionary hypotheses have turned out to be confirmed by the data. Such success is far from guaranteed. One prominent hypothesis that, for all its ingenuity, turned out to be false was the application of the Trivers-Willard theory of sex ratios to female infanticide in humans.126

The biologist Robert Trivers and the mathematician Dan Willard reasoned that even though sons and daughters are expected to yield the same number of grandchildren on average, the maximum number that each sex can promise is different. A superfit son can outcompete other males and impregnate any number of women and thereby have any number of children, whereas a superfit daughter can have no more than the maximum she can bear and nurture in her reproductive career. On the other hand a daughter is a safer bet—an unfit son will lose the competition with other men and end up childless, whereas an unfit daughter almost never lacks for a willing sex partner. It’s not that her fitness is irrelevant—a healthy and desirable daughter will still have more surviving children than an unhealthy and undesirable one—but the difference is not as extreme as it is for boom-or-bust sons. To the extent that parents can predict the fitness of their children (say, by monitoring their own health, nutrition, or territory) and strategically tilt the sex ratio, they should favor sons when they are in better shape than the competition, and favor daughters when they are in worse shape.

The Trivers-Willard theory has been confirmed in many nonhuman species and even, a bit, in Homo sapiens. In traditional societies, richer and higher-status people tend to live longer and attract more and better mates, so the theory predicts that higher-status people should favor sons and lower-status people should favor daughters. In some kinds of favoritism (like bequests in wills), that is exactly what happens.127 But with a very important kind of favoritism—allowing a newborn to live—the theory doesn’t work so well. The evolutionary anthropologists Sarah Hrdy and Kristen Hawkes have each shown that the Trivers-Willard theory gets only half of the story right. In India, it’s true that the higher castes tend to kill their daughters. Unfortunately, it’s not true that the lower castes tend to kill their sons. In fact, it’s hard to find a society anywhere that kills its sons.128 The infanticidal cultures of the world are either equal-opportunity baby-killers or they prefer to kill the girls—and with them, the Trivers-Willard explanation for female infanticide in humans.

The ultimate misogyny of female infanticide may suggest a feminist analysis, in which the sexism of a society extends to the right to life itself: being female is a capital offense. But that hypothesis doesn’t work either. No matter how sexist these societies were (or are), they did not want a world that was Frauenfrei. The men do not live in all-boy treehouses in which no girls are allowed; they depend on women for sex, children, child-rearing, and the gathering or preparation of most of their food. Families that kill their daughters want there to be women around. They just want someone else to raise them. Female infanticide is a kind of social parasitism, a free rider problem, a genealogical tragedy of the commons.129

Free rider problems arise when no one owns a common resource, in this case, the pool of potential brides. In a free market in marriages in which parents wielded property rights, sons and daughters would be fungible, and neither sex would be favored across the board. If you really needed a fierce warrior or brawny field hand around the house, it shouldn’t matter whether you raised a son for the job or raised a daughter who