For the legal system to function effectively people are generally viewed as autonomous actors able to exercise choice and responsible for their actions. It is conceivable that genetic traits associated with violent and antisocial behaviour could call into question an affected individual’s responsibility for acts of criminal violence. Evidence concerning genes associated with violent and antisocial behaviour has been introduced in criminal courts in USA and Italy, either alone or with associated environmental factors. One example of a ‘genetic defence’ is based on low levels of monoamine oxidase A (MAOA) activity, with a prevalence of around thirty percent in Caucasian males. In countries with trial by jury it is particularly relevant to consider the views of publics on criminal liability and the significance they assign to evidence citing genetic influences on behaviour. This article draws on largely qualitative research looking at participants’ explanations of, and assigning of responsibility for, violent and antisocial behaviour where environmental or genetic influences are claimed. Genetic factors were not viewed deterministically by participants but were considered by most to be irrelevant to personal responsibility. Notions of human agency, free will and choice were crucial to explanations of problem behaviours and ensured that offenders could be held responsible despite evidence on environmental and genetic factors.