One of the most prominent results of evolutionary psychology research is “the Cinderella Effect,” made famous by the work of Margo Wilson and Martin Daly (you can find one of their summary papers here). Although I don’t pretend to be an expert on the extensive literature on this phenomenon, it’s pretty much what the name connotes: the more abusive treatment by parents (usually males) of stepchildren than of their genetically related offspring. Studies have repeatedly shown that, when corrected for the proportion of stepparents among all parents, and step-children among children in a family, the step-children receive disproportionately more abuse than do genetically related children.
While these data have been disputed, I think the pattern has held up pretty well. (Anecdotally, I’ve seen this in my own family: my father’s mother died of Spanish influenza in 1918, a few months after he was born, and after his father remarried, they had another child. My father was severely mistreated compared to his half-brother. In fact, his childhood was made so unhappy by his stepmother’s abuse that he didn’t speak to his half-brother until he was about 60.)
To an evolutionary biologist, the Cinderella Effect has an obvious explanation: genetic relatedness. If you marry someone who already has a child, and then produce your own child with him or her, that natural child shares half of your genes while your stepchild shares none. Evolutionarily, it would pay you to funnel resources and solicitude toward your natural child and not the stepchild, because those resources will propagate the behavioral genes that promote such preferential care. In fact, adoption is rare among animals, probably for this very reason. Infanticide, too, has a similar explanation: when male lions take over a pride, they often kill all the cubs of the females, bringing the females back into estrus so they can produce the new males’ genetic offspring. It wouldn’t pay a male lion, evolutionarily, to take care of unrelated cubs—even though females do most of the hunting.
When I first heard about this, it made evolutionary sense to me, but I formulated an alternative hypothesis, not based on relatedness, that could explain the same thing. This hypothesis differs from the five “alternative hypotheses” described in the Wikipedia article for the Cinderella Effect.
Here’s my alternative, and I’m sure someone’s suggested it before. It’s based on convenience rather than relatedness. When one marries or mates with a previously-mated spouse, you may often do so because you love that man or woman rather than that person combined with their children. That is, it’s possible that the children of the previous mate are regarded as encumbrances rather than part of the “love package.” If you then have your own genetic children with that mate, they would represent a deliberate choice rather than an encumbrance. Ergo, because you choose your own children but not your step-children, you may treat the latter abusively. (Note that this explanation still has an evolutionary basis—you have genetic children to pass on your own genes—but the differential treatment is based not on a “relatedness module” but on culture.
My alternative theory predicts results to those of the Cinderella Hypothesis. How, then, can one discriminate among them? What you need to do is find some items that are not children, but which you can choose to acquire with your new mate versus items he or she acquired before they met you. This would separate genetic relatedness from the “acquisition” hypothesis. What items could you use?
If you marry someone who already has a pet, you might regard that pet as an encumbrance in the same way you’d regard a stepchild. You didn’t choose the pet and you may not like it very much. In contrast, if you and your mate select a pet together, you’d be more inclined, under my hypothesis, to treat it better. Ergo, the prediction of Coyne’s Encumbrance Theory is that step-pets will be mistreated or abused more than “own” pets.
Of course, there’s nothing that prevents my mechanism from operating in tandem with the genetically-based theory.
This is a bit tongue-in-cheek, but not completely. And, as I said above, I’m not deeply acquainted with the Daly and Wilson theory—though I think it’s intriguing and possibly correct—so I may have overlooked someone who’s already suggested my hypothesis.