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After a several year career as one of the second generation of women to do fieldwork in primatology, Sarah Hrdy and her husband decided to have their first child. She was already in the middle of preparations to shift her career from primatology to a subject that would allow her to do her fieldwork closer to home, with fewer long absences from home, and in a more comfortable setting to raise a baby in, namely evolutionary biology, when it occurred to her (as a mother to be) just out of personal interest to study the mothering patterns of the colony of monkeys she was observing. She knew to expect high infant mortality. Primatologists have known for over a hundred years that baby monkeys and baby apes are at extreme risk from any male other than their father. (As are baby humans.) But Hrdy was startled to discover, when she tracked the mothers of new infants carefully, that infants were at almost as much risk of murder from their own mothers as they were from unrelated male adults. This baffled her for several reasons, not least of which that while there had been a great deal of research into infanticide in primates, nobody had ever reported a case of a female primate killing her own offspring except by freakish accident. The other reason it baffled her was that, as an evolutionary biologist, she could make no sense whatsoever as to how evolution could produce individuals that destroyed their own offspring, especially among such slowly reproducing species as primates. So she contacted a few other primatologists studying other colonies of monkeys and asked them to carefully monitor the actions of new mothers ... and to their astonishment, they observed the same thing.

So she gave a preliminary paper on the subject in 1976, suggesting that more research was needed to explain how this behavior could possibly have evolved in primates, only to be interrupted in mid talk by an audience member, a prominent expert in her field. He stood up, tried to stop her from finishing reading her paper, announced that primate females absolutely do not ever murder their own children, and that if she had observed a primate colony in which primate females were killing their own children, it could only be because of something she had done to them; she must have committed some horrible breach of experimental ethics that so deranged these monkeys that she had driven them insane enough to do something that no monkey had ever done before. He then stormed out of the talk and went directly to the scientific press to denounce her for whatever it was that she had done to that monkey colony, so it probably is a good thing that she was already planning on changing fields, no?

So she quietly continued her study, working behind the scenes with other researchers while she directed her own studies towards less controversial animals, such as insects. Eventually she discovered something that appalled even her with its simplicity. Not only do mothers sometimes kill their own children, they are almost never insane when they do so. On the contrary, for a mother to murder her own child is an evolutionary adaptation without which our species would not have survived some of the environmental and social disasters of the past. What's more, the actual reasoning behind this is so simple that a straightforward simple equation in four variables is sufficient to provide a reliable estimate of the probability that any particular mother will murder any particular infant: the age of the mother, whether or not this child is the gender that the mother wanted (which, itself, turns out to be easily and universally predicted based on only two variables, the mother's social status and the predicted reliability of the food supply), the child's birth weight (and to a lesser extent other indicators of long-term viability), and her estimate of whether or not attempting to nurture this particular child will only get both her and the child killed. When she took her early estimates for this equation to the 1990 conference, she discovered that epidemiologists studying SIDS, primatologists studying infanticide (following her 1976 tip), historians digging through old records to try to quantify infanticide throughout the ages, criminologists and social psychologists trying to come up with statistical models to predict mother-on-child infanticide, and anthropologists trying to statistically analyze what variables are most consistent with cultures that have high versus low rates of infanticide, had all independently discovered the same equation. And from her viewpoint as an evolutionary biologist, Hrdy demonstrates that any sane, healthy, normal, intelligent mothers who weren't capable of coldly murdering their own infant children almost certainly had no surviving descendants at all to be our ancestors during some of the species-wide threats that have been demonstrated to have happened from the fossil record and from studies of rates of genetic drift.

I mention SIDS. One of the researchers, she says, was an epidemiologist who, in the process of trying to quantify his hunch, initiated a study in which social workers and police very, very intensively interviewed and background checked a long string of crib deaths that had been explained away as unexplained random respiratory failure. It turns out that his equation was able to predict, with high (but not absolute) reliability, which infants had actually been the victims of homicide or malign neglect. If the infant was a boy when the mother wanted a girl or vice versa, if the infant was born weighing less than 8 pounds, or if the mother was in any kind of economic or physical danger if this child survived, then the baby was doomed. His final estimate, from that initial study, was that seventy five percent of all SIDS cases are actually homicides. But, he admitted, just acknowledging this possibility puts us in an awful dilemma. To catch the 3 out of 4 women whose babies suddenly die that were actually murderers, we have to treat all SIDS cases as potential homicides, therefore piling yet more heartbreak and tragedy on the 1 out of 4 who just randomly went through the worst tragedy any family can know, the sudden and unexpected death of a beloved child. Even using the predictive equation to narrow the field of homicide investigations, we'd still be casting a very scarily public accusation of homicide on an uncomfortably large number of grieving mothers.

I also mention social psychology. The central tenet of the field of social psychology is that if under a given situation, all or nearly all individuals will engage in the same unwanted behavior, then there is less to be gained by stigmatizing those individuals and lauding the ones who don't than by studying the situation with an eye towards changing it. And you can see in a heartbeat how that applies here: if infants are at extreme risk whenever one or more of three variables are present, then we can reduce the rate of (massively under-reported, intentionally under-investigated) maternal infanticide by decreasing the economic and evolutionary pressures behind gender preference, by providing mothers with as much economic assistance and physical protection as it would require for them to feel safe providing for this baby, and by intensify supervision for the first several months of life of mothers of infants who are born weighing less than 8 pounds or looking otherwise sickly. But addressing the issue in this way, and looking into the roots of the equation that predicts maternal infanticide, makes social psychologists confront the queasy implication of all of their work: if it's that sane and natural for them to do this awful thing, if this awful thing is so hard to resist, how can we justify stigmatizing and punishing them? And if we can't, then how can we live with ourselves having just (the historian points out) joined the 85% of all known historical societies, up to and including Christian western Europe as late as the late 19th century, that socially tolerated infanticide any time in the first couple of days after birth? There's pro-choice, I mean, and then there's being so pro-choice as to join the ranks of societies that have denied the humanity of a breathing infant up to 48 hours old ... are we willing to go there? Or to at least show understanding and compassion and tolerance towards societies that did or that do? The anthropologists at the conference were especially terrified of releasing their research findings, because they knew that the accusation that a society or tribe kills children has been used to justify no shortage of genocidal invasions.

Research shows that your own mother consciously or unconsciously considered murdering you in your crib, off and on for at least the first 48 hours after your birth and not improbably for the whole first two weeks of your life, maybe even the first two months. And if your mother was under 30 when she had you and you were born male in a poor family or female in a wealthy family during times of economic hardship, or weighing less than 8 pounds, or at a time when your mother thought that her own chances of survival would improve if you didn't survive so (for example) she could get pregnant by her new husband more quickly or so she could return to work more quickly, you very nearly didn't make it. And she would have gotten away with it, too, because mothers have traditionally had a long list of potential murder weapons ready to hand, from handing you over to caretakers or adoption agencies even if she knew they had a 99% chance of killing you, to smothering you with a pillow, to switching you to infant formula that she knew was diluted with unsafe water, to declining to lift a hand to save you from some mortal peril. And because "everybody knows" that mothers don't kill their own children, nobody would have questioned her about it. Have a nice day!