The study, published in Child Development, found children with older brothers become more aggressive over time, on average, than those with older sisters. Older siblings with younger sisters become less aggressive. Those with younger brothers showed fairly stable levels of aggression over time.

Researchers looked at 451 sibling pairs, ages 9 through 18, and their parents and took into consideration such factors as parenting styles and family economics. The adolescent siblings rated their own aggressive behaviors. Trained observers assessed parental hostility directed toward each adolescent.

The researchers found parents’ hostility also played a role in the development of aggression in their children. Family economic pressure predicted increased aggression indirectly -- through its association with parental hostility.

"Understanding the factors associated with the development of aggression is essential to the design and implementation of effective intervention efforts aimed at decreasing aggression and its negative consequences," lead author Shannon Tierney Williams, of the University of California, Davis, said in a statement. "These findings suggest that such interventions may benefit from including both siblings and parents in these efforts."