Supplementary Material for: Indicated Stress Prevention addressing adolescents with high stress levels based on principles of Acceptance and Commitment Therapy – a Randomized Controlled Trial

Introduction: Stress affects many adolescents and is associated with physical and mental health symptoms that can have a negative impact on normative development. However, there are very few evidence-based, specific treatment approaches. The aim of the study was to investigate an eight-session group intervention using components of Acceptance and Commitment Therapy (ACT) enriched with elements of CBT (psychoeducation, problem solving) and art therapy, compared to a waitlist-control-group (WLC), regarding its efficacy in reducing stress and associated symptoms. Methods: We conducted a randomized controlled trial in eight cohorts. Eligible participants were 13 to 18 years old with elevated stress levels. Via block-randomization n = 70 participants were allocated to receive ACT (n = 38) or WLC (n = 32) and subsequent ACT. We used a multimodal assessment (self-reports, interviews, ecological momentary assessment, physiological markers), before treatment (T1), after the training of the ACT-group (T2) and after subsequent training in the WLC-group (T3). Primary outcome was perceived stress at T2 assessed with the Perceived Stress Scale. The trial was preregistered at the German Clinical Trials Register (ID: DRKS00012778). Results: Results showed significantly lower levels of perceived stress in the ACT group at T2, illustrating superiority of ACT compared to WLC with a medium to large effect size (d = .77). Furthermore, the training was effective in the reduction of symptoms of school burnout and physical symptoms associated with stress. Conclusion: Indicated prevention, especially when based on principles of ACT and CBT, seems efficient in significantly decreasing stress in adolescents with increased stress.