Esther Mertens

| 73 Role Personality in School-Based Intervention colleagues (2013) found stronger intervention effects on both proactive aggression and delinquent behavior for individuals with average levels of Conscientiousness and on reactive aggression for individuals with lower levels of Extraversion. Asscher and colleagues (2016) found stronger intervention effects on Conduct disorder and rule breaking behavior for individuals with higher levels of Agreeableness and Conscientiousness, but no effect of personality traits on aggression in general. In conclusion, a clear pattern of which personality traits are important to consider in interventions is lacking. Perhaps the lack of a pattern in results concerning personality traits moderating intervention effects might be explained by the type of intervention or the design of the study. Some examined interventions were school-based (e.g., Huppert & Johnson, 2010; Stoltz et al., 2013), whereas others were online (e.g., Wang et al., 2017) or in the clinical field (e.g., Asscher et al., 2016). Some examined interventions were universal (e.g., De Vibe et al., 2015; Senf & Liau, 2013), while others were selective, i.e. targeting youth who already show problembehavior (e.g., Asscher et al., 2016; Stoltz et al., 2013). Some studies focused on the intrapersonal domain (e.g., Huppert & Johnson, 2010; Senf & Liau, 2013), and others on the interpersonal domain (e.g., Asscher et al., 2016; Stoltz et al., 2013). As a first step, it is important to examine universal interventions to gain general insights in which personality traits are important moderators of intervention effects and for which, general and specific, outcomes (Spoth et al., 2006). Universal interventions are eminently suited for this aim as they often aim to address a broad range of outcomes and include a general population (Farrell, Henry, & Bettencourt, 2013). Our study adds to the literature in two ways. First, we examined the moderating role of personality traits across a broad range of competencies and problems in both the intrapersonal and interpersonal domains within a universal intervention. This enabled us to examine whether certain personality traits showed a pattern of moderating intervention effects across multiple specific competencies and problems and across both domains in general. Such a pattern would indicate that certain traits might be important to consider in interventions. Second, we analyzed the effect of personality traits on trajectories of change during the intervention. Previous research has examined personality as a moderator on a specific time point. Although this analytical procedure can show moderation of intervention effects by personality traits at that specific time point (e.g., the post or follow-up measurement), it does not show differences in change during the intervention that might be dependent on a personality trait. By examining differences in trajectories between subgroups, defined by different levels of a personality trait, we combined a person-centered approach with a variable-centered approach allowing us to analyze change within and between individuals (Greenberg & Abenavoli, 2017). In the present study, we studied personality traits as moderators of trajectories of change during a universal and widely implemented intervention, Rock & Water (R&W; Ykema, 2002, 2014). R&W is a school-based intervention aiming to 4