Meta‐analyses show that COMT Val158Met has pleiotropic effects on both executive function and emotional measures (Mier, Kirsch, & Meyer‐Lindenberg, 2009), processes that are important in relation to aggression, as well as effects on antisocial behaviour in those with ADHD (Caspi et al., 2008). This evidence concerns the gene COMT and attention deficit-hyperactivity disorder.