Esther Mertens

88 | Chapter 4 sociability required from participants and the intervention’s selective character (e.g., universal, selective, indicated). It is noteworthy that personality seems indeed somewhat more influential in the intrapersonal domain than in the interpersonal domain, both as predictor and as moderator of intervention effects. Especially intervention effects on internalizing behavior, a potentially problematic behavior in the intrapersonal domain, were affected by personality traits. Personality traits influence how one wants to feel and why (Huges, Kratsiotis, Niven, & Holman, 2020) and might therefore be more related to individuals’ own feelings, emotions, and attitudes (i.e., the intrapersonal domain), than their interactions and social perceptions (i.e., the interpersonal domain). Our findings are in line with previous research. For instance, Van Leeuwen and colleagues (2004) found that personality traits were stronger predictors of internalizing behavior than of externalizing behavior. In addition, Hughes and colleagues (2020) showed that personality traits were more strongly linked to intrapersonal coping styles than to interpersonal coping styles. Thus, one should especially be aware of potentially different intervention effects for individuals differing in levels of certain personality traits when stimulating competencies and preventing problems in the intrapersonal domain. Although we did not expect any differences in moderation effects between conditions beforehand, the different intervention conditions influenced intervention effects as well as moderation effects. We found only intervention effects in the condition in which few teachers were involved (i.e., Light condition). Even though it is often believed that intervention effects increase when more people are involved, this does not always seem to be the case. These results are supported by the meta- analysis of Durlak, Weissberg, Dymnicki, Taylor, and Schellinger (2011). Perhaps involving more people in an intervention lowers the responsibility people feel for effectively implementing the intervention, resembling a ‘bystander effect’ (Fischer et al., 2011), or results in mixed messages concerning intervention techniques (see Mertens, Deković, Van Londen, Nye, & Reitz, 2020), thereby counteracting possible intervention effects. For Extraversion, the moderation of intervention effects also differed between the conditions making these results more complex. Extraversion moderated intervention effects only in the conditions in which many people were involved in the intervention (i.e., Standard and Plus conditions). It appears that the R&W lessons alone (i.e., Light condition) were effective regardless of adolescents’ level of Extraversion. In contrast, the involvement of more people in the intervention seems to have enabled extraverted adolescents to benefit from the intervention, possibly due to their sociability, whereas it counteracted the effects of the intervention lessons for less extraverted adolescents. Again, these findings underscore the importance of considering characteristics of an intervention when evaluating its effectiveness and for whom it is effective. The present study has some strengths and limitations. Strengths of the study are the broad range of outcomes in both the intrapersonal and interpersonal domains, analyzing the effects of personality traits on change during an intervention, and