Boys who carry a particular variation of the gene Monoamine oxidase A (MAOA), sometimes called the "warrior gene," are more likely not only to join gangs but also to be among the most violent members and to use weapons, according to a new study from The Florida State University that is the first to confirm an MAOA link specifically to gangs and guns.

Findings apply only to males. Girls with the same variant of the MAOA gene seem resistant to its potentially violent effects on gang membership and weapon use.

Led by noted biosocial criminologist Kevin M. Beaver at FSU's College of Criminology and Criminal Justice, the study sheds new light on the interplay of genetics and environment that produces some of society's most serious violent offenders.

"While gangs typically have been regarded as a sociological phenomenon, our investigation shows that variants of a specific MAOA gene, known as a 'low-activity 3-repeat allele,' play a significant role," said Beaver, an award-winning researcher who has co-authored more than 50 published papers on the biosocial underpinnings of criminal behavior.

"Previous research has linked low-activity MAOA variants to a wide range of antisocial, even violent, behavior, but our study confirms that these variants can predict gang membership," he said. "Moreover, we found that variants of this gene could distinguish gang members who were markedly more likely to behave violently and use weapons from members who were less likely to do either."

The MAOA gene affects levels of neurotransmitters such as dopamine and serotonin that are related to mood and behavior, and those variants that are related to violence are hereditary. Some previous studies have found the "warrior gene" to be more prevalent in cultures that are typified by warfare and aggression.

"What's interesting about the MAOA gene is its location on the X-chromosome," Beaver said. "As a result, males, who have one X-chromosome and one Y-chromosome, possess only one copy of this gene, while females, who have two X-chromosomes, carry two. Thus, if a male has an allele (variant) for the MAOA gene that is linked to violence, there isn't another copy to counteract it. Females, in contrast, have two copies, so even if they have one risk allele, they have another that could compensate for it. That's why most MAOA research has focused on males, and probably why the MAOA effect has, for the most part, only been detected in males."

The new study examined DNA data and lifestyle information drawn from more than 2,500 respondents to the National Longitudinal Study of Adolescent Health. Beaver and colleagues from Florida State, Iowa State and Saint Louis universities detailed their findings in a paper in the journal Comprehensive Psychiatry.