A connection between the "MAO-A" gene 3R version and several types of anti-social behaviour has been found: Maltreated children with genes causing high levels of MAO-A were less likely to develop antisocial behavior. Low MAO-A activity alleles which are overwhelmingly the 3R allele in combination with abuse experienced during childhood resulted in an increased risk of aggressive behaviour as an adult, and men with the low activity MAOA allele were more genetically vulnerable even to punitive discipline as a predictor of antisocial behaviour. High testosterone, maternal tobacco smoking during pregnancy, poor material living standards, dropping out of school, and low IQ predicted violent behavior are associated with men with the low-activity alleles. The low-activity 3-repeat allele variant of the MAOA gene has also been found to occur frequently in men who join gangs. According to a large meta-analysis in 2014, the 3R allele had a small main effect on aggression and antisocial behavior, even in the absence of other interaction factors.