Prevention strategies and behavioural and pharmacological interventions for aggressive behaviour and conduct disorder are effective in some children, although a substantial number of children do not respond to prevention strategies, do not benefit from interventions or may even experience an escalation of symptom [9, 10]. One reason for this might be the heterogeneity of aggression. A second reason, which is related to the heterogeneous nature and occurrence of childhood aggressive problems, might be that children with aggressive problems often have co-occurring problems. Due to a multitude of problems, children may not respond to prevention or intervention targeting aggression, or the co-occurring problems may mask aggression, leaving it untreated. In 12 year olds, Bartels and colleagues [11] observed that at least half of the children who were deviant on aggressive behaviour (T score ≥ 67) also were deviant on rule-breaking behaviour, i.e. at least 50% of the children with clinical levels of aggression also showed a co-occurrence of clinically relevant rule-breaking behaviour. Strong links between aggression and attention-deficit/hyperactivity disorder (ADHD) [12] are often seen in the clinical presentation of ADHD [13],
