Why Would Baby Killers Still Get the Girl?

I’m thinking about killing babies. Still thinking, I guess infanticide is a better way to word it. Regardless of what we call it, the death of innocent children should strike most people as horrifying, or at least unsettling. But it happens. It’s actually an important part of life in many species13,9,16. We can see it in birds, fish, insects, rodents, carnivores, and primates. They even found fossils of a baby dinosaur in an adult dinosaur’s ribcage (Coelophysis16). Some chilling examples happen in lions12,16 and gorillas14,16. Male lions have to defeat other lions in a pride to gain their own residency. When they get in, they’ll kill infant lions. What’s shocking is what the mothers do. They actually seduce the killer lions, now a part of the pride, and they’ll have their cubs! Gorillas kill infants too, and they kill often, so much so that most female gorillas will likely experience infanticide at least once in their lifetime14,16. Much like with lions, the female often takes new protection from the killer male and subsequently births and mothers his children. If you think it’s just a male thing, sure enough, female chimpanzees kill infants too7,16. Gruesome details aside, we’re left asking why. Why kill babies?

Series of images illustrating male lion infanticide

What we should notice first is that these examples of infanticide involve mostly males killing the infants of other males. You don’t need an elaborate understanding of evolution to think, “He ‘cares’ about making his own babies. Maybe the less of someone else’s offspring, the better off are his.” This is true. Evolutionarily, my babies are more important than their babies. But the predicament of infanticide gets stickier when we turn to the behavior within the rest of the social system in which it occurs15. Sure, the killers get rid of the unrelated baby, but they also get the girl! Most can understand that species compete for survival, and even that individuals compete within a species. They have ‘selfish’ evolutionary goals. Infanticide highlights a fun nuance: the sexes within a species have goals too, and they ‘re conflicting.

Generally, infanticide pits the protection strategies versus the killers’ strategies. Female lions protect their infants. Female gorillas seek protection from dominant males. Female chimpanzees mate with many males, making basically a guessing game out of paternity. Female langurs team up with resident males in fights against attackers. Sometimes these strategies fail and when (often) females are backed into a corner, they hook up with their attacker rather than relinquish their own evolutionary goals. The villain’s babies are better than no one’s. The point is that infanticide happens in a lot of species, and it’s been happening for a long time. The strategies that we observe, killer or protection type, have survived the test of many, many generations through the benefits they provide to the strategy adopters. More important though is why these strategies ‘survive’ and what is the ultimate cause of this competition in the first place. The competition we observe isn’t arbitrary. It’s driven by genes.

Genes matter. They matter because it is genes that build our bodies, and it is genes that can copy themselves and make it to the next generation if they’re lucky. Bodies can’t make it into the next generation, but they matter in the sense that they are the immensely complex strategies that genes employ to get there. The genes that are here right now have built the kind of bodies that have lived and reproduced in the past. If genes can’t assure that copies of themselves make it into a sustainable vessel (like a body), then they won’t be in the next generation. In many cases, genes share about half the work in building bodies because their most successful replicating strategy in the past has been to build bodies they reproduce via sex. Sex halves each parent’s genetic representation. So, genes build bodies that not only strive to live, but that also want (among other things) to have sex. Each sex often has its own survival and sexual strategies. It is this reasoning that led a friend of mind to say, “This is why I love biology, because at the end of the day, everything about our biology is linked in one way or another to fucking bitches.”

Now, that’s a strong and likely overly simplistic statement, but it’s quoted for a purpose. It should cause you to question why it’s too generalizing, but also how it can be very true. If you’re a heterosexual guy, you should be thinking about the countless thoughts and behaviors of your own that are not means to end of sleeping with women. If you’re a heterosexual girl, you should be thinking about the sexually frustrated end state of my friend if he keeps saying things like that, but also how sex is not a be-all-end-all for you, your girlfriends and even a lot of guys you know. If you’re gay, then you know all of the above, and probably that sex serves so much more function outside of procreation. In fact, we know that though sex and babies are great, they’re important, we still concern ourselves with so many other things in our lives.

Yet, genes still matter. No, men aren’t lining up at sperm banks and hunting for sex with reckless abandon to get their genes the best shot in making it into the future. Women aren’t holding interviews and requesting genetic backgrounds for every man they can find just so that they can assure their own and their offspring’s survival. But here’s the thing to keep in mind: sometimes a gene’s best strategy for its future is to build a body that cares about many things seemingly counterintuitive to making it into the bedroom and pumping out children. What we think about, what we care about, and how we behave is an expression our genes and our experiences in our environment. Our genes survived in an old, harsh environment, and they made bodies that tended to survive and reproduce in it. Our appetites for food, sex, and sociality are tied to these genes, but the environment is new, and our old genetic hardware can respond differently within it. It’s often amazing what we are able to do given that our genes code for bodies with immense behavioral flexibility. Still, evidence of how we used to behave presents itself across cultures, evidence of the kinds of behavioral strategies that worked. Worked for what? Getting genes here, right now.

So it is ultimately genes, many have argued, that are at the root of why we might behave abhorrently; killing babies is morally atrocious, yes, yet it has served a function within an evolutionary framework. Our behavior aimed at others, positive or negative, is rather often a function of genetic relatedness8,10. If you haven’t felt it brewing, I want to talk about why human infanticide can be explained, in part, by genes. Like our bodies, our brains have been designed by natural selection to act in accordance with time-tested (many, many generations) strategies to maximize survival and reproductive benefits in light of the costs. Anything that promotes you and your babie’s health, happiness and survival is an evolutionary benefit. Unfortunately, unrelated babies that impinge on these benefits can be considered a cost. But so can your own babies. Women actually kill their own babies more than men2,9.

How can this be? Baby survival is a benefit. However, baby rearing is a cost, and baby survival is literally fruitless if baby doesn’t reach reproductively able adulthood due to illness, deformities, or lack of adequate resources, all of which predict whether mothers kill their infants4,10. Interestingly, when children get older, the likelihood of being killed by their parents decreases, presumably due to investment. Generally, when children are killed at any age, it is mostly likely to be by a stepparent. A single stepparent in the home is the best predictor of ‘filicide’, and preschool children are 40 to 100 times more likely to be killed by stepparents than by genetically related parents4,10. Across cultures, children receive less care, less attention and less resources from stepparents than from genetically related parents5,11,1. Children living with stepparents are also abused more than children with biological parents, as one study showed that 120 of 10,000 children 4 years and younger were the victims of abuse, whereas 3 in 10,000 children of biological children were abused3,1.

We should take relief in the fact that most children are not abused or killed by their parents, related or not. You can also take weird refuge in the conclusion of these controversial, hard-hitting authors, Martin Daly and Margo Wilson, that infanticide is not necessarily an evolved adaptation. It could likely be the case that it’s just easier to care about one’s own offspring, and whatever evolved psychology that keeps one from investing in unrelated offspring can sometimes lead to, well, ugly consequences. I say it’s weird to take solace in this conclusion because even if it were the case that human infanticide was adaptive, that’s all it would be: natural selection acting on a behavior. It would be ‘natural’ to kill unrelated infants, but it would not be right or ethical or moral, for much the same reasons that killing an adult would be wrong. No one is arguing that infanticide is O.K. (well, some people do6). What I do want you to consider is the role that genes play in the trends we observe in human and animal behavior. Behaviors like infanticide are icky, but when you ‘count the genes’, you can make testable predictions using condition-dependent strategies that worked in an old environment, and that are leaking out in current behavior (in one form or another) in our new environment. We can count genes to help us better understand behavior, good or bad. And, hopefully, with understanding, comes compassion. Don’t kill babies.


  1. Alcock, J. (2001). The triumph of sociobiology. Oxford: Oxford University Press.
  2. Bourget, D., Grace, J., & Whitehurst, L. (2007). A review of maternal and paternal filicide. Journal of the American Academy of Psychiatry and Law, 35, 74-82.
  3. Daly, M., & Wilson, M. (1985). Child abuse and other risks of not living with both parents. Ethology and Sociobiology, 6, 197-210.
  4. Daly, M., & Wilson, M. (1988). Homicide. New York: A. de Gruyter.
  5. Flinn, M. V., Leone, D. V., & Quinlan, R. J. (1999). Growth and Fluctuating Asymmetry of Stepchildren. Evolution and Human Behavior, 20, 465-479.
  6. Giubilini A, Minerva F. Journal Medical Ethics (2012). doi:10.1136/medethics-2011-100411
  7. Goodall, J., & Goodall, J. (1990). Through a window: my thirty years with the chimpanzees of Gombe. Boston: Houghton Mifflin.
  8. Hamilton, W. D. (1964). The genetical evolution of social behaviour, I and II. Journal of Theoretical Biology, 7, 1-52.
  9. Hausfater, G., & Hrdy, S. B. (1984). Infanticide: comparative and evolutionary perspectives. New York: Aldine Pub. Co..
  10. Liddle, J. R., Shackelford, T. K., & Weekes-Shackelford, V. A. (2012). Why Can’t We All Just Get Along? Evolutionary Perspectives on Violence, Homicide, and War. Review of General Psychology, 16(1), 24-36.
  11. Marlowe, F. (1999). Male Care and Mating Effort among Hadza foragers. Behavioral Ecology and Sociobiology, 46, 57-64.
  12. Packer, C., & Pusey, A. (1983). Adaptations of Female Lions to Infanticide by Incoming Males. American Naturalist, 121, 716-728.
  13. Parmigiani, S., & Saal, F. S. (1994). Infanticide and parental care. Chur, Switzerland: Harwood Academic Publishers.
  14. Watts, D. P. (1989). Infanticide in Mountain Gorillas: New Cases and a Reconsideration of the Evidence. Ethology, 81, 1-18.
  15. Wrangham, R. W. (1979). On the Evolution of Ape Social Systems. Social Science Information, 18, 335-368.
  16. Wrangham, R. W., & Peterson, D. (1996). Demonic males: apes and the origins of human violence. Boston: Houghton Mifflin.

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