Yet another very astute reader has proposed a way to adjudicate between the two competing hypotheses on why stepfathers kill their children.

In an earlier post, I explain that there are now two competing hypotheses about why stepfathers are more likely to kill their children.  Initially, Martin Daly and Margo Wilson, the two Deans of Modern Evolutionary Psychology, proposed the “discriminative parental solicitude” theory of filicides (killing of children by parents).  They argue that, because stepchildren do not carry the stepfathers’ genes, their survival and reproductive success cannot increase the stepfathers’ own reproductive success.  Worse yet, any resources invested in the stepchildren only take away from resources that could be invested in their own genetic children.  So stepfathers should be evolutionarily (if largely unconsciously) motivated to underinvest, neglect, and sometimes kill their stepchildren.

More recently, a team of Swedish zoologists, led by Hans Temrin, has proposed an alternative theory.  They suggest that stepfathers may be criminal and violent to begin with, even before they become stepfathers.  The logic of assortative mating would suggest that only men of low mate value would marry women of low mate value (older women with children from previous marriages).  Men of low mate value are more likely to be violent and criminal, and they are therefore more likely to kill children, both stepchildren and their own genetic children.  Temrin et al. present initial evidence in support of their new theory in their study of filicides in Sweden.

Now, a long-time reader of my blog from Portugal, Mr. Leonardo Castro, has offered a very insightful way to adjudicate between these two hypotheses empirically.

Mr. Castro begins by noting that sons require more parental investment from the father than daughters do.  As I explain in an earlier post, this is why the presence of a son deters divorce and the subsequent departure of the father from the family. Daughters’ future reproductive success will depend mostly on their youth and physical attractiveness, so, once they are born, there is very little that fathers (or mothers) can do to increase their fitness, beyond keeping them alive and healthy. Sons’ future reproductive success, on the other hand, will depend heavily on their resources and status. So the fathers (and mothers) must make sure that their sons will inherit whatever resources and status they have, so that their sons can acquire as many resources and as high a status as the parents can assure them to have.

From the perspective of the stepfather, the presence of a stepson will therefore be more of a hindrance to the reproductive success of his own genetic son than the presence of a stepdaughter.  A stepson will compete with the genetic son for the resources and status that the stepfather would want his own genetic son to have, but a stepdaughter would not.  Nor would a stepdaughter compete with a genetic daughter, because there is nothing for either to compete for.  So a stepfather should be more motivated to murder and maim stepsons than stepdaughters, that is, if discriminative parental solicitude is the evolutionary motive for their behavior.

So Mr. Castro hypothesizes that the proportion of sons should be higher among the murder victims of stepfathers than among the murder victims of genetic fathers.  If so, then that would support Daly and Wilson’s discriminative parental solicitude explanation of filicides.  In contrast, if Temrin et al. are correct, then the proportion of sons among the murder victims of stepfathers should not be any higher (or lower) than among the murder victims of genetic fathers.

Mr. Castro’s logic also leads to another set of predictions.  If discriminative parental solicitude is the mechanism, then stepfathers who have a genetic son should be more likely to kill their stepson than stepfathers who only have genetic daughters.  If Temrin et al.’s explanation is correct, then stepfathers with genetic sons should be no more or no less murderous of their stepsons than stepfathers with only genetic daughters.

Mr. Castro on his own located a Canadian study of filicides (Harris, G. T., Hilton, N. Z., Rice, M. E., & Eke, A. W.  (2007).  Children killed by genetic parents versus stepparents.  Evolution and Human Behavior, 28, 85-95).  In this study, the proportion of sons among the murder victims of stepfathers is 56% and the proportion of sons among the murder victims of genetic fathers is 57%.  The difference is not statistically significant, and is in the wrong direction from that predicted by discriminative parental solicitude.  So the initial inspection of Harris et al.’s data seems to support Temrin et al.’s alternative explanation of filicides.  Harris et al.’s study does not present information on whether the stepfathers have genetics sons or daughters to allow us to test the second set of predictions.

At any rate, I believe Mr. Castro’s idea provides a novel way to adjudicate between Daly and Wilson’s discriminative parental solicitude and Temrin et al.’s alternative explanation of filicides.

As an aside, it is interesting to note that, just like Mr. Alok Lal, who has made a significant contribution to my blog earlier, Mr. Castro was also trained as an engineer and currently works in the computer field.  What is it about engineers who work with computers and their insight into evolutionary psychology?  My guess is that general intelligence trumps both interest and experience.  Engineers on average tend to have higher intelligence than many others.  And they appear to be able to apply their intelligence to solving theoretical problems in other fields, like evolutionary psychology, better than others who were initially more interested in human behavior and who thus may have studied psychology or social sciences in college.

About the Author

Satoshi Kanazawa

Satoshi Kanazawa is an evolutionary psychologist at LSE and the coauthor (with the late Alan S. Miller) of Why Beautiful People Have More Daughters.

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