Eighty percent of U.S. children have a sibling. Siblings can be sources of companionship, play, and joy—as well as jealousy, rivalry, and aggression. A national survey found that one-third of U.S.

We all know brothers and sisters often fight, but when does sibling rivalry cross the line? When does “picking on your little brother or sister” become bullying? We do research on bullying in schools.