Among the "just-so stories" popular with scientists who seek genetic explanations of human behavior, few are so odious as the idea that males are genetically predisposed to kill their stepchildren.

The idea is that such behavior would have been adaptive for our Stone Age ancestors. Males who carried genes pushing them to kill their stepchildren, goes the theory, would have left more kids themselves because the murder would have freed up their new mate for, well, mating (a female nursing a small child is much less likely to conceive). Also, a Stone Age man who cared for and supported only his biological children, rather than stepkids, would leave more descendants than a man who cared for his stepchildren. Such murderous males would have left more descendants than males who tolerated, let alone supported, their stepchildren; we, their descendants, would therefore also carry the stepchildren-killing gene.