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Background Although positive psychology interventions (PPIs) show beneficial effects on mental health in non-clinical populations, the current literature is inconclusive regarding its effectiveness in clinical settings. We aimed to examine the effects of PPIs on well-being (primary outcome), depression, anxiety, and stress (secondary outcomes) in clinical samples with psychiatric or somatic disorders. Methods A systematic review and meta-analysis was conducted following PRISMA guidelines. PsycINFO, PubMed, and Scopus were searched for controlled studies of PPIs in clinical samples between Jan 1, 1998 and May 31, 2017. Methodological quality of each study was rated. We used Hedges’ adjusted g to calculate effect sizes and pooled results using random-effect models. Results Thirty studies were included, representing 1864 patients with clinical disorders. At post-intervention, PPIs showed significant, small effect sizes for well-being (Hedges’ g = 0.24) and depression (g = 0.23) compared to control conditions when omitting outliers. Significant moderate improvements were observed for anxiety (g = 0.36). Effect sizes for stress were not significant. Follow-up effects (8–12 weeks), when available, yielded similar effect sizes. Quality of the studies was low to moderate. Conclusion These findings indicate that PPIs, wherein the focus is on eliciting positive feelings, cognitions or behaviors, not only have the potential to improve well-being, but can also reduce distress in populations with clinical disorders. Given the growing interest for PPIs in clinical settings, more high quality research is warranted as to determine the effectiveness of PPIs in clinical samples. Trial registration PROSPERO CRD42016037451 Electronic supplementary material The online version of this article (10.1186/s12888-018-1739-2) contains supplementary material, which is available to authorized users.

Acceptance and Commitment Therapy (ACT) has been demonstrated to be effective in reducing depressive symptoms. However, little is known how and for whom therapeutic change occurs, specifically in web-based interventions. This study focuses on the mediators, moderators and predictors of change during a web-based ACT intervention. Data from 236 adults from the general population with mild to moderate depressive symptoms, randomized to either web-based ACT (n = 82) or one of two control conditions (web-based Expressive Writing (EW; n = 67) and a waiting list (n = 87)), were analysed. Single and multiple mediation analyses, and exploratory linear regression analyses were performed using PROCESS and linear regression analyses, to examine mediators, moderators and predictors on pre- to post- and follow-up treatment change of depressive symptoms. The treatment effect of ACT versus the waiting list was mediated by psychological flexibility and two mindfulness facets. The treatment effect of ACT versus EW was not significantly mediated. The moderator analyses demonstrated that the effects of web-based ACT did not vary according to baseline patient characteristics when compared to both control groups. However, higher baseline depressive symptoms and positive mental health and lower baseline anxiety were identified as predictors of outcome across all conditions. Similar results are found for follow-up. The findings of this study corroborate the evidence that psychological flexibility and mindfulness are distinct process mechanisms that mediate the effects of web-based ACT intervention. The results indicate that there are no restrictions to the allocation of web-based ACT intervention and that web-based ACT can work for different subpopulations. Netherlands Trial Register NTR2736 . Registered 6 February 2011.