New research shows that aggressive sibling behavior can affect children as much as peer bullying.

Sibling rivalries will exist until the end of time, but, according to new research, what was once primarily seen as a right of passage may leave behind more than just broken toys.

Researchers at the University of New Hampshire (UNH) say that aggression between siblings can create significantly worse mental health outcomes in children, to the point that it mirrors the effects of being bullied by a peer at school.

According to research published in the forthcoming issue of the journal Pediatrics, about 32 percent of children report being victims of sibling bullying in the past year.

Children younger than 9 experienced the greatest mental distress—showing signs of depression, anxiety, and anger—but all age groups were affected in some way. Sibling aggression included being hit or kicked without injury, stealing or breaking a toy on purpose, or saying something that made another sibling feel bad, scared, or unwanted.

Lead study author Corinna Jenkins Tucker, an associate professor of family studies at UNH, said that even one instance of sibling bullying could affect a child’s mental health, showing that sibling aggression is not benign, regardless of how severe or frequent it is.

She said the data shows even small events can have a big impact on some siblings, but “in the same way not everyone who smokes gets lung cancer,” not everyone responds the same way to sibling conflict.

“Just to keep in mind, we looked at it at a group level,” she said in an interview with Healthline. “Siblings are going to have disagreements regardless, but there are more constructive ways to handle them and destructive ways to handle them.”

Jenkins Tucker and other researchers at UNH’s Crimes Against Children Research Center reached their conclusion by analyzing data from a sample of 3,599 children, ages 1 month to 17 years, in the National Survey of Children’s Exposure to Violence. The data is collected through interviews with parents and children.

Considering the outcome of their research, the authors suggest that anti-bullying programs should address sibling bullying as well.

“If siblings hit each other, there’s a much different reaction than if that happened between peers,” Jenkins Tucker said. “It’s often dismissed, seen as something that’s normal or harmless. Some parents even think it’s beneficial, as good training for dealing with conflict and aggression in other relationships.”