Satoshi Kanazawa reports that a fundamental tenet of evolutionary psychology may be doubtful. Specifically, the fact that children are far more likely to be murdered in a home with a stepparent (read: stepfather) may require a different interpretation than standard evolutionary psychology theory has given it.
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.
According to a recent study that Kanazawa discusses, it turns out that stepfathers themselves are much more likely to have a history of criminal violence, and thus a propensity to murder stepchildren may be as much or more due to the criminal proclivities of these men than to the fact that the children are stepchildren. Furthermore, it appears that violent men are equally likely to kill their own children as they are to kill stepchildren. Kanazawa explains why stepfathers are more likely to have criminal records:
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.
However, Kanazawa doesn't draw the obvious conclusion here: the fact that divorced women with children have lower mate value, and that therefore they are more likely to become married to men with criminal histories, is itself a perfect example of a result that agrees with evolutionary theory. Thus, while the cited study calls into question one of the foundational facts of evolutionary psychology, the study itself confirms another fundamental tenet, namely that the value of a mate can be measured by such things as the ability to provide offspring, the presence of non-related offspring, and a history of criminal violence.