Cinderella effect

Cinderella effect is the name for a phenomenon found in psychology: Child abuse and mistreatment is more common in families with stepparents. 
It is named after the fairy tale Cinderella, where a girl gets mistreated by her stepsisters and stepmother. Psychologists think this shows a bias towards kin. A person has fewer incentives to invest in a child that is not his or her direct offspring. 
P.D. Scott first used the term Cinderella effect, in a study he did in 1973: He studied 29 cases of babies who were beaten to death in anger; he found that in 15 cases, the baby's stepfather was the killer.