BACKGROUND: Allelic variation of the monoamine oxidase A (MAOA) gene has been 
implicated in conduct disorder and antisocial, aggressive behavior in humans 
when associated with early adverse experiences. We tested the hypothesis that a 
repeat polymorphism in the rhesus macaque MAOA gene promoter region influences 
aggressive behavior in male subjects.
METHODS: Forty-five unrelated male monkeys raised with or without their mothers 
were tested for competitive and social group aggression. Functional activity of 
the MAOA gene promoter polymorphism was determined and genotypes scored for 
assessing genetic and environmental influences on aggression.
RESULTS: Transcription of the MAOA gene in rhesus monkeys is modulated by an 
orthologous polymorphism (rhMAOA-LPR) in its upstream regulatory region. High- 
and low-activity alleles of the rhMAOA-LPR show a genotype x environment 
interaction effect on aggressive behavior, such that mother-reared male monkeys 
with the low-activity-associated allele had higher aggression scores.
CONCLUSIONS: These results suggest that the behavioral expression of allelic 
variation in MAOA activity is sensitive to social experiences early in 
development and that its functional outcome might depend on social context.