Esther Mertens

: ) Skills and struggles in the intra- and interpersonal domain are important predictors of adolescents’ psychosocial wellbeing and their development into adulthood. The intra personal domain indicates one’s feelings, emotions, and attitudes about the self. The inter personal domain indicates the ability to build and maintain positive relationships with others, to understand social situations, roles and norms, and to res- pond appropriately. A powerful context to address these two domains is the school context. The overall aim of the current dissertation was to study whether and under what circumstances universal interventi- ons in secondary schools can successfully stimulate students’ competencies and prevent problems in the intra- and inter- personal domain. I conducted a meta-analysis to increase insights in these interventions in general and a Randomized Controlled Trial to gain a detailed understanding of a speci c intervention, Rock andWater (R&W). Findings indicate that universal school-based interventions generally showed small positive e ects in both the intra- and interpersonal domain. Intervention e ects of R&W were comparable in magnitude, especially in the intrapersonal do- main, and appeared to be dependent of intervention dosa- ge, suggesting ‘ less is more ’, and students’ personality traits. Furthermore, results of the meta-analysis implied that some components associated with stronger intervention e ects were rarely implemented, while some components related to weaker e ects were often implemented. Together these results indicate that there is potential to improve the e ecti- veness of universal school-based interventions.