The so-called “Cinderella Effect” is one of the foundational empirical findings in evolutionary psychology. Martin Daly and the late Margo Wilson, the two Deans of Modern Evolutionary Psychology, discovered, in their analysis of homicide data from Canada and Detroit, that stepchildren, those who live with a stepparent (usually, a stepfather), are anywhere from 40 to 100 times as likely to be murdered or maimed as those who live with two biological parents in the household.

In retrospect, this makes perfect sense. Parental love for children is evolutionarily conditional on the children’s ability to increase the parents’ reproductive success. Stepchildren do not carry any of the of the stepparents, so there is absolutely no evolutionary reason for stepparents to love, care for and invest in their stepchildren. Worse yet, any resources invested in stepchildren take away from investment that the stepparents could make in their own genetic children. So, in the cold, heartless calculus of evolutionary logic, it makes perfect sense for the stepfather to kill his stepchildren, so that his mate (the mother of the stepchildren) will only invest in their joint children, children whom the stepfather has had with the mother and who carry his genes. Only they can increase the stepfather’s reproductive success.

Daly and Wilson call this process “discriminative parental solicitude.” It’s just an academic way of saying that parents play favorites. Parental love for children is not unconditional, and is proportionate to the children’s expected reproductive success, which in turn increases the parents’ reproductive success. The higher the expected reproductive success of the children, the more the parents love them and invest in them. This is why parents always (but usually unknowingly) favor physically more attractive, healthier, and more intelligent children over physically less attractive, sicklier, and less intelligent children. And since stepchildren, no matter how attractive or intelligent, can never contribute to the reproductive success of the stepparents, there is absolutely no evolutionary reason for stepparents to love and care for them.

This finding, and the evolutionary psychological explanation for why stepchildren face greater risk of murder and injury, has been part of the foundational core of modern evolutionary psychology for its entire history. It is discussed in every single introductory textbook on evolutionary psychology as one of the early major empirical and theoretical triumphs of evolutionary psychology, including my own book Why Beautiful People Have More Daughters. But now a team of Swedish researchers (Hans Temrin, Johanna Nordlund, Mikael Rying, and Birgitta S. Tullberg), in their forthcoming paper in Current Zoology, have put this firm conclusion in doubt.

In their paper, Temrin et al. do not question that stepchildren are more likely to be killed and maimed by their stepfathers; they only question discriminative parental solicitude as the explanation for it. They point out, and empirically demonstrate with a small Swedish sample, that men who become stepfathers, by marrying women who already have children from previous unions with other men, are more likely to be criminal and violent to begin with. And Temrin et al. argue that their greater tendency toward criminality and violence, not their genetic unrelatedness, is the reason they are more likely to kill and injure their stepchildren.

Once again, in retrospect, this makes perfect sense. women with children are on average older, so they have lower mate value than younger women without children. Given choice, and all else equal, all men would prefer to marry younger women without children rather than older women with children with other men. The logic of assortative would suggest that women with lower mate value are more likely to mate with men with lower mate value. And, as I explain in an earlier post, men with lower mate value are more likely to be criminal and violent.

In their analysis, Temrin and his colleagues show that men who are in stepfamilies – men who have married older women with children – are significantly more likely to have criminal records, both for crimes in general and for violent crimes. And in cases where stepfathers kill children in their family, they are equally likely to kill their genetic children as they are to kill their stepchildren. (Of course, given the ubiquity of cuckoldry – especially in Sweden! – and thus paternity uncertainty, they are not necessarily real genetic children; they are only putative genetic children.)

The only major weakness of Temrin et al.’s study, which the authors themselves openly acknowledge, is their extremely small sample, taken from one small nation. There just aren’t many homicides in Sweden, child homicides or otherwise. So their findings must be replicated, with larger samples and in other societies. But, at the very least, their paper has begun to throw one of the foundational principles of evolutionary psychology into possible doubt. In my experience, this is the first and (so far) the only study ever to do so. If their findings are replicated, and if their explanation for the greater risk of homicide faced by stepchildren is true, then Hans Temrin and his colleagues have secured their places in the Evolutionary Psychology Hall of Fame for their act of successful academic regicide.