Fights between siblings should not be thought of as just "part of growing up," researchers from the University of New Hampshire said.
The mental health of children suffered when they encountered "sibling aggression" as much as it did from peer aggression.
"Even kids who reported just one instance had more mental health distress," said Corinna Jenkins Tucker, lead author of the study published in the July issue of Pediatrics that analyzed data on 3,599 children one month to 17 years old.
When it came to physical violence, like being hit by a brother or sister, younger children had greater "mental health distress" than adolescents. But all the children were equally affected when their siblings stole their toy or broke it on purpose, or said mean things to them.