Sudden gains have been shown to be a common phenomenon and an important predictor of outcome in psychotherapy (Tang & DeRubeis, 1999). The present meta-analysis examined the growing literature on sudden gains in psychotherapy, their effects on outcome, and moderators of these effects. We searched PsycINFO, PubMed, ProQuest Dissertations & Theses, Scholar and Web of Science databases for articles between January 1st 1999 and May 31st 2019 and included 50 studies (n = 6355) in the meta-analysis. Sudden gains were found to significantly predict outcome at post-treatment (g = 0.68) and follow up (g = 0.61) above and beyond treatments, disorders and settings. Studies using Tang and DeRubies's (1999) criteria for identification of sudden gains had smaller effect sizes compared to studies using altered criteria. Pretreatment severity levels did not significantly predict the effects of sudden gains and neither did treatment setting (randomized controlled trials vs. naturalistic settings). Finally, number of sessions and sudden gains' reversal rates were both negatively associated with the effect sizes of sudden gains. These findings suggest that sudden gains are a ubiquitous phenomenon in psychotherapy but their effects may be moderated by a number of factors. Research and clinical implications are discussed.

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