Abstract: A total of three studies involving 144 eligible participants were included in the review. Two of the studies (56 participants) compared an early version of acceptance and commitment therapy (ACT) with CBT, and one study (88 eligible participants) compared extended behavioural activation with CBT. No other studies of third wave CBT were identified. The two ACT studies were assessed as being at high risk of performance bias and researcher allegiance. Post-treatment results, which were based on dropout rates, showed no evidence of any difference between third wave CBT and other psychological therapies for the primary outcomes of efficacy (risk ratio (RR) of clinical response 1.14, 95% confidence interval (CI) 0.79 to 1.64; very low quality) and acceptability. Results at two-month follow-up showed no evidence of any difference between third wave CBT and other psychological therapies for clinical response (2 studies, 56 participants, RR 0.22, 95% CI 0.04 to 1.15). Moderate statistical heterogeneity was indicated in the acceptability analyses (I2 = 41%). Very low quality evidence suggests that third wave CBT and CBT approaches are equally effective and acceptable in the treatment of acute depression. Evidence is limited in quantity, quality and breadth of available studies, precluding us from drawing any conclusions as to their short- or longer-term equivalence. The increasing popularity of third wave CBT approaches in clinical practice underscores the importance of completing further studies to compare various third wave CBT approaches with other psychological therapy approaches to inform clinicians and policymakers on the most effective forms of psychological therapy in treating depression.

Plain-language summary: In this review, we focused on third wave CBT approaches, a group of psychological therapies that target the process of thoughts (rather than their content, as in CBT) to help people become aware of their thoughts and accept them in a non-judgemental way. The aim of the review was to find out whether third wave CBT was more effective and acceptable than other psychological therapy approaches for people with acute depression. The review included three studies, involving a total of 144 people. The studies examined two different forms of third wave CBT, consisting of acceptance and commitment therapy (ACT) (two studies) and extended behavioural activation (BA) (one study). All three studies compared these third wave CBT approaches with CBT. The results suggested that third wave CBT and CBT approaches were equally effective in treating depression. However, the quality of evidence was very low because of the small number of studies of poor quality that we included in the review; therefore it is not possible to conclude whether third wave CBT approaches might be more effective and acceptable than other psychological therapies in the short term or over a longer period of time. Given the increasing popularity of third wave CBT approaches in clinical practice, further studies should be prioritised to establish whether third wave CBT approaches are more helpful than other psychological therapies in treating people with acute depression.