One gene called MAOA that played an especially strong role has been shown in other studies to affect antisocial behavior - and it was disturbingly common, the team at the University of North Carolina reported.

People with a particular variation of the MAOA gene called 2R were very prone to criminal and delinquent behavior, said sociology professor Guang Guo, who led the study. "I don't want to say it is a crime gene, but 1 percent of people have it and scored very high in violence and delinquency," Guo said in a telephone interview.

His team, which studied only boys, used data from the National Longitudinal Study ofAdolescent Health, a U.S. nationally representative sample of about 20,000 adolescents in grades 7 to 12. The young men in the study are interviewed in person regularly, and some give blood samples.

Guo's team constructed a "serious delinquency scale" based on some of the questions the youngsters answered. "Nonviolent delinquency includes stealing amounts larger or smaller than US$50, breaking and entering, and selling drugs," they wrote in the August issue of the American Sociological Review.

"Violent delinquency includes serious physical fighting that resulted in injuries needing medical treatment, use of weapons to get something from someone, involvement in physical fighting between groups, shooting or stabbing someone, deliberately damaging property, and pulling a knife or gun on someone."

They found specific variations in three genes - the monoamine oxidase A (MAOA) gene, the dopamine transporter 1 (DAT1) gene and the dopamine D2 receptor (DRD2) gene - were associated with bad behavior, but only when the boys suffered some other stress, such as family issues, low popularity and failing school.

MAOA regulates several message-carrying chemicals called neurotransmitters that are important in aggression, emotion and cognition such as serotonin, dopamine and norepinephrine.

The links were very specific. The effect of repeating a grade depended on whether a boy had a certain mutation in MAOA called a 2 repeat, they found. And a certain mutation in DRD2 seemed to set off a young man if he did not have regular meals with his family.

"But if people with the same gene have a parent who has regular meals with them, then the risk is gone," Guo said.

"Having a family meal is probably a proxy for parental involvement," he added. "It suggests that parenting is very important."