Esther Mertens

146 | Chapter 7 General discussion The focus of this dissertation was on the effectiveness of universal school-based interventions aiming to improve competencies and to prevent the development of problems in the intrapersonal (e.g., psychological wellbeing) and interpersonal (e.g., aggression) domain. The first aim of this dissertation was to study the effectiveness of these universal school-based interventions (i.e., what works). By conducting a Randomized Control Trial (RCT), I evaluated the effectiveness of a specific universal school-based intervention, Rock andWater (R&W; Ykema, 2002; 2018), for prevocational students (Chapter 3). The effectiveness of universal school-based interventions in general I examined by conducting a meta-analysis (Chapter 6). The second aim was to study whether heterogeneity in the context and in the student population affected intervention effectiveness (i.e., under what circumstances and for whomdoes it work?). More specifically, I focused on the extent to which intervention dosage, that is, the intervention’s ecological width (i.e., the number of involved systems in an intervention) and intervention’s time span (Chapter 3), and students’ personality traits (Chapter 4) affected intervention effectiveness. The third aim was to study working mechanisms of universal school-based interventions (i.e., how does it work?). As mechanism of change (i.e., mediator), I examined whether classmates’ modeling and reinforcement mediated the effect of R&W on the peer context in the classroom (an aspect of the interpersonal domain; Chapter 5). In the meta-analysis, I analyzed which components of interventions were associated with intervention effects in the intra- and interpersonal domains of universal school-based interventions in general (Chapter 6). In this chapter, I summarize and discuss the main findings, elaborate on strengths and limitations, make recommendations for future research and point out practical implications. Aim 1: Effectiveness of Universal School-Based Interventions The present dissertation is in line with previous research suggesting that prevocational students have an increased risk of developing psychosocial problems (e.g., Harakeh, De Looze, Schrijvers, Van Dorsselaer, & Vollebergh, 2012; Stevens & De Looze, 2018). My results of the RCT illustrated that during these two years prevocational students in the Control condition had a tendency to develop minor psychosocial problems, particularly in the interpersonal domain. More specifically, the students slightly decreased in psychological wellbeing (i.e., intrapersonal domain) and interpersonal relations in the class, and slightly increased in aggression, bullying, and victimization (i.e., interpersonal domain). Apparently, school policy as usual (i.e., Care As Usual; CAU) is not effective enough to prevent the development of psychosocial problems for prevocational students at this age (12 – 14 years). This finding emphasizes the necessity