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.

"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.

monoamine oxidase A is an enzyme that breaks down important neurotransmitters in the brain, including dopamine, norepinephrine, and serotonin. The enzyme is regulated by monoamine oxidase A gene (MAOA). Humans have various forms of the gene, resulting in different levels of enzymatic activity. People with the low-activity form (MAOA-L) produce less of the enzyme, while the high-activity form (MAOA-H) produces more of the enzyme.

Several studies have found a correlation between the low-activity form of MAOA and aggression in observational and survey-based studies. Only about a third of people in Western populations have the low-activity form of MAOA. By comparison, low-activity MAOA has been reported to be much more frequent (approaching two-thirds of people) in some populations that had a history of warfare. This led to a controversy over MAOA being dubbed the “warrior gene.”