Esther Mertens

70 | Chapter 4 Abstract Individuals with higher or lower levels of certain personality traits might benefit more from an intervention than individuals with opposite levels of these traits, as some personality traits could make individuals more vulnerable to develop problems, providing more potential to improve, whereas other personality traits could facilitate transfer of the learned skills to daily life. The aim of the present study was to examine whether Big Five personality traits affected the effectiveness of a universal school-based intervention aiming to improve competencies and prevent problems in adolescents’ intra- (e.g., psychological wellbeing) and interpersonal (e.g., aggression) domains. In a two-year randomized controlled trial, adolescents ( N = 1299, M age  = 12.38) reported on the outcomes at four different time points. Parents reported at baseline on adolescents’ personality. Although most intervention effects were not moderated by personality traits, three patterns of moderation emerged. First, there was a tendency that more vulnerable adolescents – based on their levels of certain personality traits such as high levels of Extraversion, high levels of Agreeableness, high levels of Neuroticism, or low levels of Conscientiousness – benefitted most from the intervention. Second, high levels of Extraversion appeared to enable adolescents to benefit more from a universal intervention when it requires sociability from participants. Third, personality traits seemed to affect the intrapersonal domain more than the interpersonal domain, both as predictors and as moderators of intervention effects. The present study increases general insights in how personality traits might affect intervention effects for certain types of adolescents, interventions, and outcomes. Keywords : Personality; RCT; School-based intervention; Moderation; Intrapersonal domain; Interpersonal domain