If you  smash your fist into someone’s body are you being aggressive and anti-social, or are you just doing what your genes programme you to do? The idea of genes affecting behaviour isn’t new and isn’t disputed, but the use of certain genes as a mitigating factor in criminal behaviour is a contentious issue. Genes have been used as part of the defence in cases of murder, and moral and ethical arguments around this use have focused on the low validity of the research evidence plus the generally accepted concepts of free will and personal responsibility. The argument has been taken a stage further now as a convicted murderer in Italy has had his sentence reduced partly because of his history of psychiatric illness and also partly because his genome includes five genes known to be associated with violent behaviour. One of these genes is a variant of MAOA, which codes for an enzyme which breaks down amines in the brain, and this low-activity variant correlates in research findings with violence and aggression, giving it its nickname the “warrior” gene. However, as we all know, correlations are not necessarily causal; and then there is the responsibility debate. So, what would you decide if you were on the jury, the defendant was clearly guilty of murder, but also had a gene profile predisposing him or her to violence and aggression?