Bad genes may make a brute, but it takes a bad environment to make a bully.

A new study shows that it's bad influences more so than genes that are behind making children bullies.

Researchers say the results show that the type of social aggression that makes young girls and boys hurt each other with social snubs and snobbery may have very different roots than the type of physical aggression involved in picking fights.

Read Web MD's "Girls Just Wanna Be Mean."

Seeking Out the Source of Bullying

In the study, researchers analyzed the roots of social and physical aggression in a group of 234 6-year-old twins.

Researchers had the girls' peers and teachers rate their physical and social aggressiveness, and their results showed that the twins' shared genes explained most of the differences in physical aggression, but relatively few of the differences in social aggression.

For example, shared genes accounted for about 50-60 percent of the variation in physical aggression. But genetics only accounted for about 20 percent of the differences in social aggression.

Instead, shared or unique environmental factors appeared to explain most of the variation in social aggression (60 percent).

Researchers say it's the first study to show that social aggression seems to be determined to a lesser extent by genetic factors and to a greater extent by environmental factors than physical aggression.

The study also showed that children reported to have high levels of physical aggression were more likely to become socially aggressive.

Researchers say those findings support the notion that aggressive behaviors passed down through the genes may be initially expressed in physical means, such as throwing punches, and later replaced by socially aggressive behaviors, such as bullying.

"Whether and when this developmental shift occurs, however, may depend on the extent to which the child is exposed to an environment that specifically promotes the use of social aggression," says researcher Mara Brendgen of the University of Quebec in Montreal, in a news release.

"Our results have important implications for preventive interventions," she says, "as they suggest that reducing physically aggressive behavior at an early age might also help prevent the development of social aggression in young children." she says.

Read Web MD's "Too Much TV May Make Kids Bullies."

Read Web MD's "Half of Kids Are Bullied, Study Suggests."

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Brendgen, M. Child Development, July/August 2005; vol 76. News release, Society for Research in Child Development.