Boys who carry a particular variation of the gene Monoamine oxidase A (MAOA), called the 'warrior' gene by some, are more likely to join gangs and be among their most violent members, according to a study from a Florida State University criminologist that associates MAOA to gangs and guns.

Findings apply only to males, which makes an unsubstantiated allele argument necessary.  Girls with the same variant of the MAOA gene don't show any propensity toward gang membership or weapon use.   MAOA  has also been implicated in ADHD, bipolar disorder, cancer and smoking.  Basically, if you don't have any other explanation for something, MAOA is the way to go.

"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 biosocial criminologist Kevin M. Beaver of FSU's College of Criminology and Criminal Justice.

"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 can affect 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 even said the 'warrior' gene is 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 to be published in a forthcoming edition of the journal Comprehensive Psychiatry. Currently, the paper ("Monoamine oxidase A genotype is associated with gang membership and weapon use") is accessible online at via the "Articles in Press" link. 

In addition to the MAOA study, Beaver has also published research linking  genetics to adolescent victimization, formation of delinquent peer group, use of steroids and "roid rage".