COMT Val158Met has been observed to predict aggression/extreme antisocial behaviour in individuals with ADHD (Caspi et al., 2008; Langley et al., 2010; Monuteaux et al., 2009; Salatino‐Oliveira et al., 2012) across six independent studies and in a meta‐analysis (Caspi et al., 2008) with the association applying to the subtype of aggression in those with ADHD, not to aggressive behaviour in the general population (Caspi et al., 2008). Here, COMT is linked to attention deficit-hyperactivity disorder.