Some maltreated children develop into adults with antisocial behavior and become violent offenders, whereas many others do not. In the August 2 Science Caspi et al. provide a genetic insight into the variable response to childhood maltreatment (Science 2002, 297:851-854). They examined a functional polymorphism in the promoter of the gene encoding monoamine oxidase A (MAOA), an enzyme involved in the metabolism of neurotransmitters. Deficiencies in MAOA have been linked to aggressive behavior in mice and men. They studied more than 1,000 children, examining a VNTR (variable number tandem repeat) polymorphism in the MAOA promoter that is known to affect expression levels. They found evidence for a significant gene-environment interaction; maltreated male children with low MAOA expression showed the greatest risk of developing antisocial behaviour. This study demonstrates how the interplay between genotype and environmental factors can affect complex adult behaviours.