Cinderella Effect Theory
Full article at wikipedia.org/cinderella_effect
In evolutionary psychology, the Cinderella effect is the alleged higher incident of different forms of child-abuse and mistreatment by step and adoptive parents 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.
The United States indeed incur greatly elevated risk of child maltreatment of various sorts, especially lethal beatings". Studies have found that not biologically related parents are up to a hundred times more likely to kill a child than biological parents.