A warrior gene could make teenagers more likely to join gangs, scientists believe.
Boys who carry the gene are likely to be dangerous, violent and carry weapons, according to the research.
The gene affects levels of the brain chemicals dopamine and serotonin - which affect behaviour and mood.
But the gene defect has no effect on girls, it emerged.
Scientists have previously linked the gene to anti-social behaviour but have not been able to confirm the link with gangs and guns until now.
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It is also more common for boys from warring or aggressive cultures with the variation of the Monoamine oxidase A (MAOA) gene, to join gangs, it was found.
Biosocial criminologist Kevin M. Beaver, from Florida State University's College of Criminology and Criminal Justice, who led the study, said that the research sheds new light on the interplay of genetics and environment.
