Aggression in adolescents is influenced by siblings Children who’ve older brothers are more aggressive over time, on average, than those people who have older sisters. Older siblings with more youthful sisters become less intense. Children with older sisters who are very aggressive are more aggressive and older siblings with more youthful brothers showed fairly stable levels of aggression over time www.med-drugs.com . In sum, the current presence of both old and more youthful siblings influences the development of intense behavior in adolescence. Having a brother or a highly intense sibling of either gender can result in greater boosts in aggression as time passes. These findings are from researchers at the University of California, Davis, and so are published in the September/October 2007 issue of the journal Kid Development.