Esther Mertens

116 | Chapter 6 interventions that did not include practice. Similarly, Cuijpers (2002) concluded in his review of school-based drug interventions that interventions using more active methods (e.g., discussion) were more effective than interventions using more passive methods (e.g., didactic instruction). Regarding structural components, the general assumption is that longer and extensive interventions aremore effective than briefer and less extensive interventions (Yeager & Walton, 2011). The evidence, however, is conflicted. Some meta-analyses showed that longer and extensive interventions are indeed more effective. For instance, interventions showed stronger effects as the time span, number of sessions, and involved persons (i.e., whole school, parents) increased (De Vries et al., 2015; Ttofi & Farrington, 2011). Other meta-analyses, on the other hand, showed that briefer and less extensive interventions are more effective (i.e., “less is more”). For instance, interventions showed stronger effects when the time span was short, the number of sessions limited, and no additional services were provided (Cuijpers, 2002; Kaminski et al., 2008; Van der Put et al., 2018). Longer and extensive interventions require more time and effort to implement, which potentially takes away time and energy from the main goal (Bakermans-Kranenburg, Van IJzendoorn, & Juffer, 2003). Therefore, we hypothesized that briefer and less extensive interventions would be related to stronger intervention effects on students’ intra- and interpersonal domains than longer and extensive interventions. In summary, identifying components related to stronger or weaker intervention effects has important theoretical and practical implications. First, it expands our knowledge concerning interventions. We begin to unravel what is more important to change students’ intra- and interpersonal domains: What they learn, how they learn it, or how the intervention is set up? Second, it enables schools to make informed decisions about which intervention to implement and existing and new interventions can be optimized; the present meta-analysis not only examined what interventions “should do”, but also what they “should not do.” This knowledge is a first step towards improving the effectiveness of school-based interventions addressing students’ intra- and interpersonal domains, ultimately enhancing students’ positive development, psychosocial wellbeing, and preparedness for future challenges. Method Inclusion and Exclusion Criteria We sought to include evaluations of universal secondary school-based interventions targeting students’ intrapersonal and interpersonal domains. Universal secondary school-based interventions were defined as interventions delivered to students during