Siblings uniquely promote the development of sympathy and a quality relationship with a brother or sister increases levels of altruism in teens, a new study has found.

Researchers said the findings are surprising because boys typically report that they benefit less than girls from peer relationships.

"In our study, most relationships were not as important for boys as they were for girls," said study co-author Laura Padilla-Walker from Brigham Young University.

"But the sibling relationship was different - they seemed to report relying on sibling affection just as much as girls do. It's an area where parents and therapists could really help boys," she said.

Padilla-Walker and fellow Brigham Young University professor Jim Harper found that siblings uniquely promote the development of sympathy. A quality relationship with a brother or sister also increased teens' levels of altruism, also known as prosocial behaviour.

"Having a sibling you can count on seems to make a difference especially for prosocial behaviour. Best friends make a contribution, but siblings still matter," said Harper.

The BYU researchers followed 308 pairs of teenage siblings for three years. The project measured their development and tracked the quality of their relationships with friends and family members.

"This was the first siblings study to control for all these other important relationships. We can say that siblings are uniquely important, which is encouraging," Padilla-Walker said.

Boys who have a hostile relationship with a sibling were significantly more likely to have behavioural problems later on, the study found. The study appears in the Journal of Research on Adolescence.