OBJECTIVE: Preclinical and human family studies clearly link monoamine oxidase A (MAOA) to aggression and antisocial personality (ASP). The 30-base pair variable number tandem repeat in the MAOA promoter regulates MAOA levels, but its effects on ASP in humans are unclear. METHODS: We evaluated the association of the variable number tandem repeat of the MAOA promoter with Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, ASP disorder (ASPD) traits in a community sample of 435 participants from the Hopkins Epidemiology of Personality Disorders Study. RESULTS: We did not find an association between the activity of the MAOA allele and ASPD traits; however, among whites, when subjects with a history of childhood physical abuse were excluded, the remaining subjects with low-activity alleles had ASPD trait counts that were 41% greater than those with high-activity alleles (P < .05). CONCLUSION: The high-activity MAOA allele is protective against ASP among whites with no history of physical abuse, lending support to a link between MAOA expression and antisocial behavior.