Some traditions have a series of milestones, such as traditional Judaism, which grants full legal personhood to a baby only after it has survived thirty days.
If I have tried to make infanticide a bit more comprehensible, it is only to reduce the distance between the vast history in which it was accepted and our contemporary sensibilities in which it is abhorrent. But the chasm that separates them is wide. Even when we acknowledge the harsh evolutionary logic that applies to the hard lives of premodern peoples, many of their infanticides are, by our standards, hard to comprehend and impossible to forgive. Examples from Daly and Wilson’s list include the killing of a newborn conceived in adultery, and the killing of all a woman’s children from a previous marriage when she takes (or is abducted by) a new husband. And then there are the 14 percent of the infanticidal justifications on the list that, as Daly and Wilson point out, do not easily fall into categories that an evolutionary biologist would have predicted beforehand. They include child sacrifice, an act of spite by a grandfather against his son-in-law, filicides that are committed to eliminate claimants to a throne or to avoid the obligations of kinship customs, and most commonly, the killing of a newborn for no other reason than that she is a girl.
Female infanticide has been put on the world’s agenda today by census data revealing a massive shortage of women in the developing world. “A hundred million missing” is the commonly cited statistic for the daughter shortfall, a majority of them in China and India.118 Many Asian families have a morbid preference for sons. In some countries a pregnant woman can walk into an amniocentesis or ultrasound clinic, and if she learns she is carrying a girl, she can walk next door to an abortion clinic. The technological efficiency of daughter-proofing a pregnancy may make it seem as if the girl shortage is a problem of modernity, but female infanticide has been documented in China and India for more than two thousand years.119 In China, midwives kept a bucket of water at the bedside to drown the baby if it was a girl. In India there were many methods: “giving a pill of tobacco and bhang to swallow, drowning in milk, smearing the mother’s breast with opium or the juice of the poisonous Datura, or covering the child’s mouth with a plaster of cow-dung before it drew breath.” Then and now, even when daughters are suffered to live, they may not last long. Parents allocate most of the available food to their sons, and as a Chinese doctor explains, “if a boy gets sick, the parents may send him to the hospital at once, but if a girl gets sick, the parents may say to themselves, ‘Well, we’ll see how she is tomorrow.’ ”120
Female infanticide, also called gendercide and gynecide, is not unique to Asia.121 The Yanomamö are one of many foraging peoples that kill more newborn daughters than sons. In ancient Greece and Rome, babies were “discarded in rivers, dunghills, or cesspools, placed in jars to starve, or exposed to the elements and beasts in the wild.”122 Infanticide was also common in medieval and Renaissance Europe.123 In all these places, more girls perished than boys. Often families would kill every daughter born to them until they had a son; subsequent daughters were allowed to live.
Female infanticide is biologically mysterious. Every child has a mother and a father, so if people are concerned about posterity, be it for their genes or their dynasty, culling their own daughters is a form of madness. A basic principle of evolutionary biology is that a fifty-fifty sex ratio at sexual maturity is a stable equilibrium in a population, because if males ever predominated, daughters would be in demand and would have an advantage over sons in attracting partners and contributing children to the next generation. And so it would be for sons if females ever predominated. To the extent that parents can control the sex ratio of their surviving offspring, whether by nature or by nurture, posterity should punish them for favoring sons or daughters across the board.124
One naïve hypothesis comes out of the realization that it is the number of females in a population that determines how rapidly it will grow. Perhaps tribes or nations that have multiplied themselves to the Malthusian limit on food or land kill their daughters to achieve zero population growth.125 One problem for the ZPG theory, however,