Some studies have shown that high-activity MAOA alleles are related to ADHD (Domschke et al., 2005; Xu et al., 2007) and impulsive personality traits (Manuck et al., 2000) owing to dopamine deficiency caused by increased activation of the MAOA enzyme (Hwang et al., 2018). Here, MAOA is linked to attention deficit-hyperactivity disorder.