This will come as little surprise to most parents: Children who have older brothers tend to become more aggressive than those with older sisters, according to a new study.

The study generated a host of nuanced findings:

  • Having a brother or a highly aggressive sibling of either gender can lead to greater increases in aggression over time.
  • Older siblings with younger sisters tend to end up less aggressive.
  • Older siblings who were aggressive tended to have younger siblings who were also aggressive, and vice versa.?

Parental hostility, in some cases linked to economic pressure, also played a role in the development of kids' aggression.

The study, led by Tierney Williams of the University of California, Davis, is published in the September/October 2007 issue of the journal Child Development. Williams and colleagues looked at 451 sibling pairs, ages 9 through 18, and their parents. Siblings each rated their own aggressive behaviors, and parents described economic pressures on the family, such as difficulty paying bills. Trained observers assessed the hostility the parents directed toward each adolescent during family interactions.

The research was supported by the National Institute of Child Health and Human Development, the National Institute on Drug Abuse, and the National Institute of Mental Health.