By Eric Crampton 24/11/2015


I think this is the first time I’ve seen evolutionary biology featured in a Press piece on crime. The piece notes the disproportionate number of children in New Zealand killed by step-fathers.

Why stepfathers kill their lovers’ small children but spare their own has troubled Canadian evolutionary psychologist Martin Daly for decades.

He and his late wife Margo Wilson founded the Cinderella theory in the 1980s, researching the deaths of 700 Canadian children.

What they found suggested the unconditional love a parent feels for a screaming child who has soiled their nappy, is not innate for a stepparent – and makes them more likely to lash out.

… Building on Darwin’s theory of evolution, the relationship between the new man on the scene and his lover’s child is forged by biological altruism, Daly and Wilson found.

That means humans, like other animals, are programmed to investing their time into reproducing their own genes – not someone else’s – and sometimes that resentment becomes deadly.

…”My argument, in psychology – and it’s the same with those other animals who engage in step-parenting – is the step-parent is doing it as a courting step.

“People love their own children more than they love someone else’s child. That’s not to say they don’t love them… [but] generally, they’re not going to throw themselves in front of a truck for them.”

In the Canadian research, birth parents overwhelmingly smothered or shot their children, and a third of fathers committed murder-suicide.

Stepfathers usually beat children to death and just 1 in 67 killed themselves too, Daly and Wilson found.

They note that the New Zealand data for a proper test would be hard to come by (as is all NZ data about everything because of because).

Satoshi Kanazawa explained it this way:

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 genes 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.

But he also cites some contrary evidence from Sweden suggesting that the background characteristics of stepdads do a lot of the work.

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. Divorced 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 mating 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.

So a proper New Zealand study would want to correct for the stepfathers’ ex ante characteristics. It would also then partially answer Jan Pryor’s question of the theory, raised in the original Press piece:

The theory also did not explain whether solo-mothers living financially strained lifestyles were targeted by men who preyed upon them and their children, Pryor said.

I’m curious how much of the effect here works through the biological Cinderella story and how much works through the assortative mating dynamics at the lower tail of the distribution.

Add to the list of “open questions that could be answered by somebody with time to muck around in IDI applications and the Stats Data Lab”, or “Masters theses waiting to be written”..