ADHD is the most prevalent neuropsychiatric disorder in childhood, as it occurs in 3-7% of school-age children, and displays by high heritability of 60-80% [14], which means that it has a strong genetic background.

Its main symptoms are restlessness, fidgetiness, lack of attention and concentration as well as increased impulsivity. The latter is also a hallmark of ADHD in adulthood, and persistence of the disorder can be found in approximately 65% of affected children [15].

Of the two main symptom domains, inattentiveness and hyperactivity/impulsivity, our own studies show the latter to strongly share genetic liability with oppositional behaviour [16]. The hyperactive-impulsive ADHD subtype is associated with aggression [17] and goes along with a highly increased risk for aggressive behaviour [18]. Especially impulsive aggression is frequently observed in ADHD [19], potentially mediated by difficulties with control of behaviour and emotional dysregulation (irritable and aggressive moods) [20] [21].

Treatment of aggression with stimulant medication has been investigated in children with ADHD and demonstrated to be effective, with an average effect size around 0.8 [22] [23] [24]. Not surprisingly, the presence of co-morbid CD further facilitates aggressive behaviour. Accordingly, an ADHD polygenic score was recently shown to be significantly associated with co-morbid CD, which is explained by aggression symptoms [25].

This suggests that aggression in ADHD indexes genetic and clinical severity. Increased aggression in ADHD opens up a negative developmental trajectory: increased ADHD frequency is observed in detained adolescents [26] [27] and in young adult male prison inmates, as much as 45% were found to suffer from ADHD [28].

Our own research found a 6-fold increase in violent and aggressive incidents among male prisoners associated with current ADHD, even after controlling for antisocial personality disorder [29]. Importantly, anti-ADHD medication in adulthood results in a significant reduction in the criminality rate [30], which was 4-fold higher in ADHD compared to controls. This underscores the importance of ADHD diagnosis and treatment in aggression and criminal behaviour.