Cinderella effect.
In evolutionary psychology, the Cinderella effect is the alleged higher incidence of different forms of child-abuse and mistreatment by stepparents than by biological parents. It takes its name from the fairy tale character Cinderella. Evolutionary psychologists describe the effect as a remnant of an adaptive reproductive strategy among primates where males frequently kill the offspring of other males in order to bring their mothers into estrus, and give the male a chance to fertilize her himself. There is both supporting evidence for this theory and criticisms against it.