It's not nearly as overt as a hand sign or a coloured bandana, but DNA may offer one clue as to whether someone belongs to a gang or not.
Males with a particular form of gene called MAOA are twice as likely to join a gang, compared to those with other forms, finds a new study of more than 2000 US teens. What's more, gang members with these mutations are far more likely to use a weapon than other members.
"For the most part, people haven't really thought of the biological or genetic underpinnings to gang membership, says Kevin Beaver, a biosocial criminologist at Florida State University in Tallahassee, who led the study.
The relatively common mutations result in reduced levels of a protein, called monoamine oxidase A, which recycles several of the chemicals that foster neuron connections.
Low MAOA activity has been linked previously to antisocial behaviour in people who experienced child abuse. While two brain regions involved in perceiving and controlling emotions are shrunken in people with no history of criminality or abuse who have the mutation.
To determine whether an environment besides an abusive childhood could elicit MAOA's connection to violence, Beaver's team looked at the genotypes of 1155 females and 1041 males who participated in a long-term study of adolescent health that covered the period 1994 to 2002.
During two rounds of interviews, participants indicated whether they had been in a gang in the past year and whether they had ever used a weapon in a fight.
Overall, 42 per cent of males possessed the low activity form of MAOA, and about 5 per cent of all males said they had joined a gang. However males with the low activity form were twice as likely to join a gang as those with the high activity form.
Similarly, males with the low activity form were about twice as likely to have used a weapon as other teens. While male gang members with the same mutations were four times as likely as other members to wield a weapon.
"This gene is predicting gang membership, but it's really predicting it for the very violent gang members," Beaver says.
Nonetheless, he cautions against over-interpreting his team's results. "It doesn't mean that everyone with this particular allele is going to be violent and is going to become a gang member – or vice versa," he says.
Rather, in communities where gangs are common, people with low MAOA activity could be slightly more likely to join a gang than others, Beaver says.
However, given the high prevalence of these mutations and low rates of gang membership, genetic counselling to identify youth at an increased risk of joining gangs would be unfeasible.
"We're not going to change someone's DNA, but we can alter the environment which would, in turn, blunt that genetic effect," Beaver adds.