Reducing Aggression in Adolescence: Identifying Effective Behaviour Change Techniques

Peer aggression among adolescents is a problem in many countries. It has negative short- and long-term consequences for both perpetrators and victims. Many interventions to reduce aggression have been designed and evaluated. However, which components make interventions more effective are still unknown. This thesis aims to identify the most effective behaviour change techniques (BCTs) to reduce aggression among adolescents in order to optimise interventions.
First, a multi-level meta-analysis of 101 trials was conducted to identify the most effective BCTs for (a) interventions targeted to adolescents at higher risk of being aggressive and (b) (universal) interventions addressed to all adolescents. Action planning was identified as the most effective BCT for targeted interventions and problem solving and behavioural practice were the most effective BCTs in universal interventions. It was difficult to extract the independent effect of these BCTs as most of the interventions included in the meta-analysis were composed of several BCTs. Therefore, two randomised controlled trials were conducted to examine the effect of (a) action planning with at-risk adolescents and (b) problem solving with a general sample of adolescents. First, 100 referred adolescents were randomised to complete a volitional help sheet for anger management or to an active control group. No effect on aggression was found for the full sample, but the intervention was effective for adolescents with high callous-unemotional traits. Second, 908 adolescents were randomised to a brief online problem-solving intervention or a passive control group. This intervention was effective in reducing verbal aggression, but not other types of aggression.
The research in this thesis identified the most effective BCTs to reduce aggression among adolescents and showed that brief interventions using only those BCTs are effective in some circumstances. However, future research should investigate which combinations of techniques are the most effective in reducing overall aggression.