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When it comes to reproduction, though, nothing is quite so easy. Nature abhors deficits and surpluses, and successful parenting often involves spacing births far enough apart so no offspring go hungry but not so far apart that resources go unused. Many mammals nurse a pup or cub or child far longer than necessary because lactation shuts down ovulation and a new pregnancy can be put off until circumstances are right for it. Only when they are will the mother conceive again. "All mother mammals are forced to make the most of the resources available while making trade-offs compatible with their own survival," says Hrdy.
But what happens when this resource-management system breaks down? What if the mother gambles wrong and an offspring comes along when there are no resources to support it, or what if she is too young to care for it? What if the infant is sickly and seems likely to languish no matter how well it's looked after? Humans agonize over these situations, but mothers throughout the animal kingdom show a surprising willingness to abandon or even kill such luckless young rather than pour energy down a bottomless reproductive well.
An American black bear, which normally gives birth to two or three cubs at a time, may walk away from one born alone, calculating that it's better to wait for a multiple birth next year than exhaust herself with a singleton now. Mice will examine offspring after they're born and eat undersize young, improving the overall fitness of the litter and giving themselves a valuable dose of protein in the process.
Things become truly troubling when human beings facing the same difficult circumstances start making equally brutal choices. Anthropologists report that during a three-year period of social upheaval in Bolivia in the 1930s, nearly every mother in an Ayoreo Indian village committed infanticide at least once. In India and China, selective infanticide of baby girls is still commonly, if quietly, practiced. And while there may be less of a history of such killing in the U.S., periodic cases of high-schoolers secretly giving birth and then murdering their infant are proof that such practices know no cultural boundaries.
If an impulse to commit infanticide is indeed part of our genetic legacy, can it be forgiven? Or does such a crime remain a crime no matter how strong the primal drives behind it? Anthropologists take the long view--at least when the crime is abandonment and not murder. "I think we have to re-examine the harsh penalties we place on young, uneducated women who abandon infants," says anthropologist Helen Fisher of Rutgers University. "They were dancing to primitive, natural rhythms, and they got out of synch."
The problem with such scientific forgiveness is that it may give human beings, with their much celebrated free will, too little credit. Most mothers will love and nurture even an unwanted infant because, well, it seems the right thing to do. Most will stretch their resources to the breaking point in order to have a second child not because that's what their genes drive them to do but because they love having children. Critics of the work of Hrdy and others resist drawing too many parallels between human and animal parents, insisting that the few traits that distinguish us from other species are far more important than the many things we share.