Esther Mertens

| 53 Effectiveness of a Psychophysical Intervention Interpersonal domain Interpersonal relations in the class. Perceived interpersonal relations in the class were assessed using the Classroom Peer Context Questionnaire (Boor-Klip, Segers, Hendrickx, & Cillessen, 2016). The questionnaire measures negative social exchanges between classmates, the extent to which students feel comfortable around their classmates, and the unity and inclusiveness among classmates. The questionnaire consists of 12 items (e.g., “In this class students like each other.”) answered on a 5-point Likert-type scale (1 =  totally not true to 5 =  completely true ). Some items were recoded so that high scores indicatedmore positive interpersonal relations in the class (Cronbach’s α = .80 - .85). Externalizing behavior. To measure the presence of externalizing problems, students completed the externalizing subscale of the short version of the YSR (Chorpita et al., 2010). The subscale consists of 6 items (e.g., “I argue a lot.”) rated on a 3-point Likert-type scale (0 = never to 2 = often ; Cronbach’s α = .65 - .79). Aggression. Students’ reactive and proactive aggression was measured with the Reactive and Proactive Aggression Questionnaire (Dodge & Coie, 1987). The questionnaire consists of 6 items (e.g., “If they tease me, I get angry.”) answered on a 5-point Likert-type scale (1 = never to 5 = almost always ; Cronbach’s α = .65 - .83). Bullying and victimization. To assess the frequency of bullying and experienced victimization, students completed the 2 global items of the Olweus Bully/Victim Questionnaire (Solberg, & Olweus, 2003). The items were: “How often have you taken part in bullying others?” and “How often have you been bullied?” and were preceded by a definition of bullying. Students responded on a 5-point Likert-type scale (1 = never to 5 = almost always ). Secondary Outcomes Self-control. To assess students’ ability to control their impulses and interrupt undesired behaviors, students completed the short version of the Self-Control Scale (Finkenauer, Engels, & Baumeister, 2005) consisting of 11 items (e.g., “I am good at resisting temptation.”) rated on a 5-point Likert type scale (1 = not at all to 5 = very much ). Some items were recoded so that high scores indicated high levels of self- control (Cronbach’s α = .62 - .72). Self-reflection. Students completed the subscale Engage in reflection of the Self- Reflection and Insight Scale (Sauter, Heyne, Blöte, Van Widenfelt, & Westenberg, 2010) to measure the extent to which students inspect and evaluate personal thoughts, feelings, and behaviors. The subscale consists of 6 items (e.g., “I often think about how I feel about something.”), preceded by a definition of self-reflection, answered on a 6-point Likert scale (1 = disagree strongly to 6 = agree strongly ). Some items were recoded so that higher scores represented higher levels of self-reflection. Reliability 3