In Italy, a judge reduced the sentence of a defendant by 1 year in response to evidence for a genetic predisposition to violence. The best characterized of these genetic differences, those in the monoamine oxidase A (MAOA), were cited as especially relevant. Several months previously in the USA, MAOA data contributed to a jury reducing charges from 1st degree murder (a capital offence) to voluntary manslaughter. Is there a rational basis for this type of use of MAOA evidence in criminal (...) court? This paper will review in context recent work on the MAOA gene–environment interaction in predisposing individuals to violence and address the relevance of such findings to murder trials. Interestingly, the MAOA genetic variants impact future violence and aggression only when combined with the adverse environmental stimuli of childhood maltreatment. Thus nature and nurture interact to determine the individual’s risk. Based on current evidence, I argue there is a weak case for mitigation. But should future experiments confirm the hypothesis that individual differences in impulse control and response to provocation found in MAOA-L men (without abuse) are significantly magnified when combined with childhood maltreatment, the case could turn into a stronger one. (shrink)