Stay Cool Kids?! Effectiveness, Moderation and Mediation of a Preventive Intervention for Externalizing Behavior

Stable externalizing behavior in childhood places children at risk for the development of a chronic and persistent pattern of externalizing behavior problems. Preventive interventions that aim to interrupt this developmental trajectory are crucial. Until now, no evidence-based intervention for children with elevated levels of externalizing behaviors existed in the Dutch School context. First, we conducted a meta-analysis on international studies of school-based indicated interventions and found that individually delivered interventions are effective in reducing externalizing behavior (d = .28), increasing prosocial behavior (d = .43) and changing social cognitive functioning (d = .82). Next we examined the effectiveness of Stay Cool Kids, a preventive intervention for externalizing behavior. Moreover, we tested for whom and how the intervention works. This study used a cluster Randomized Controlled Trial with 48 elementary schools (264 fourth-grade children with elevated levels of externalizing behavior)in the Netherlands, that were randomly assigned to the intervention or no-intervention control condition. Results showed that Stay Cool Kids reduced reactive and proactive aggressive behavior (mean effect sizes on aggressive behavior were .22, .29, .25 and .21 according to children, teachers, mothers, and fathers, respectively). Moreover, the effect of the intervention was clinically significant, because participation in the intervention resulted in differences in the child’s functioning from clinical to sub-clinical or normative levels of externalizing behavior according to teachers. Some aspects of problems in social cognitive functioning were reduced (approval of aggression, d = .22) and children showed more positive self-perception (d = .49). The results of follow-up assessment showed that Stay Cool Kids resulted in sustained reduction in teacher reported aggression, when children moved to fifth grade. Results from the moderation analyses showed that boys, immigrant Dutch children, children who are moderately conscientious (organized and planful) and less extraverted (sociability and agency) benefitted more from the intervention on specific outcomes at short-term. In addition, less extraverted children showed stronger intervention effects immediately after the intervention, and effects at follow-up were stronger for these children. Tests of the theoretical model of Stay Cool Kids, proposing that changes in social cognitions would lead to changes in behavior, revealed that an increase in positive self-perception resulted in a decrease in child aggressive behavior. Child participation in the intervention resulted in an increase in maternal involvement, which in turn resulted in decreases in aggression. Altogether, the studies in this dissertation show that Stay Cool Kids can reduce externalizing behavior (reactive and proactive aggression, clinical externalizing behavior) across settings (school and home), in boys and girls and in children with different ethnic backgrounds. Effect sizes of Stay Cool Kids were similar to findings in international meta-analyses and can be considered as small to medium. It is important to realize that (maintenance of) effects of the intervention were not the same for all children. These results underline the importance of personalized mental health care adapted to children’s specific needs