Are stepparents more likely to kill their children?

Whereas young children incurred about seven times higher rates of physical abuse in step-plus-genetic-parent home than in two-genetic-parent homes, the differential in fatal abuse was on the order of 100-fold.

Source: “Violence Against Stepchildren” from Current Directions in Psychological Science (June 1996)

This is a well documented phenomenon known as The Cinderella Effect. It’s been called “one of the poster-children of evolutionary psychology.”