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…