health problems—and specifically attention-deficit hyperactivity disorder (ADHD) symptoms—if they had been victims of abuse, but fewer problems if they had not, compared with boys with the high-MAOA-activity genotype. In another longitudinal study, this one of 514 adolescent twin boys aged 8–17 years, Foley et al.18 found that childhood adversity—based on parent and child report—predicted a 3-month history of conduct disorder (DSM-III, Diagnostic and Statistical Manual (of mental disorders)) differently for children with and without the low-activity-MAOA allele. Once again, boys with the low-MAOA-activity allele were more likely to be diagnosed with conduct disorder if exposed to higher levels of childhood adversity and less likely if exposed to lower levels of adversity, compared with boys with the high-MAOA-activity allele. Similar results emerged in Nilsson et al.'s19 cross-sectional investigation of 81 adolescent boys when the predictor was psychosocial risk, operationalized in terms of maltreatment experience and living arrangement. Only boys with the low-MAOA-activity allele were affected by such risk, such that those with a history of adversity engaged in more criminal behavior (composite of vandalism, violence, stealing) and those lacking this history engaged in less.
