Studies reported in English or Spanish were included according to the following a priori criteria: (1) design: randomized (RCTs) or pseudo-randomized (pRCT) controlled trials; (2) participants: patients with a history of at least one Major Depressive Episode (MDE); (3) intervention: studies had to test the efficacy of a specific Mindfulness-Acceptance-Based Behavioral Therapy or technique (e.g. cognitive defusion, values clarification), which are widely recognized in the literature as acceptance and mindfulness strategies or techniques; (4) comparator: treatment as usual (TAU) or other active intervention for the treatment of depression; (5) outcome measures: studies had to include one or more valid measures of ruminative thoughts: such as the Rumination on Sadness Scale (RSS; Raes, Hermans, & Eelen, 2003), the Rumination Response Style Questionnaire (RRS or RSQ; Nolen-Hoeksema & Morrow, 1991), or the Rumination and Reflection Questionnaire (RRQ; Trapnell & Campbell, 1999).

Excluded studies were reviews, editorials, non-randomized, uncontrolled or observational studies, case series and cases studies. We also excluded studies that presented repeat analyses of data already available in the included studies, as well as those published in a language other than English or Spanish.

A systematic literature search was performed in the following electronic databases: Embase (Elsevier), Medline and PreMedline (OvidSP), PsycInfo (EbscoHost), Cinahl (EbscoHost), and the Cochrane Controlled Trials Register, from the first available year to December 2016. The search strategy was developed for each electronic database using the combination of the following Medical Subject Heading (MeSH) and free-text terms: ([acceptance and commitment therapy] OR [acceptance-based behavior therapy] OR [acceptance-based strategies] OR [acceptance-based treatments] OR [acceptance-based interventions] OR [acceptance-based therapies] OR [acceptance-based approaches] OR [ACT] OR [mindfulness] OR [mindfulness-based strategies] OR [mindfulness-based treatments] OR [mindfulness-based interventions] OR [mindfulness-based therapies] OR [mindfulness-based approaches] OR [MBCT] OR [MBSR] OR [third wave therapies] OR [third wave of CBT]). The search strategies were developed, tested and refined by an experienced information specialist with input from authors (full details can be made available on request). In addition, reference lists of selected articles and other systematic reviews were inspected, and leading researchers in the field of MBCT were contacted to identify further relevant studies. Initially, duplicates were removed from the total number of identified records. Abstracts from the remaining records were then screened to retrieve full-text articles for assessment of eligibility. Finally, studies fulfilling inclusion criteria were selected for meta-analytic assessment. The retrieval process was verified by the four authors (AR, JB, LP, WP). Study inclusion was performed by the authors and disagreements were resolved by consensus.

A data extraction sheet was developed and the data collection included author and year, country of analysis, setting, study design (RCT or pRCT), sample characteristics (including age, sex, type of sample, condition), group characteristics (including intervention, comparison condition, number of group participants, drop-outs and withdrawals, follow-up), primary outcomes, definitions of effectiveness, and main findings. For each eligible study, all relevant data were extracted by one researcher. To ensure the quality and accuracy of the data, a second researcher verified the extracted data with their original sources. Quality assessment of studies was performed by one reviewer and verified for accuracy by a second reviewer using the criteria developed by the NHS Center for Reviews and Dissemination, which include 25 items for assessing the internal and external validity (Centre for Reviews and Dissemination, CRD), 2006). Disagreements between raters were resolved by discussion.

A meta-analysis was performed with the RevMan 5.2 software. We used the standardized mean difference (Hedges’ g) between treatment and control/comparison groups in the scores’ change from baseline to post-intervention (a negative value represents an effect favoring the MBCT intervention). Since only one study reported the standard deviations (SD) of change, we used the baseline average value as a proxy of the pooled SD, as recommended by Morris (2008). The cumulative result was then calculated by weighting the effect of each study by the inverse of the variance. We used a random effect model since we assumed different true effects across studies.

Statistical heterogeneity was analyzed by means of the Cochrane's Q and Higgins’ I2 statistics (Deeks, Altman, & Bradburn, 2008) (a conservative value of 0.10 was established for the significance of Q). A sensitivity analysis, excluding one study at a time was performed to assess its effect on results. Risk of publication bias was assessed by means of the visual inspection of the funnel plot and the Egger's test (Egger, Davey Smith, Schneider, & Minder, 1997).

Methodological quality of the included studies is shown in Appendix A. The two pRCT were consider of poor quality whereas all the RCT were classified as having a satisfactory quality. Regarding randomization, Schoenberg and Speckens (2014) allocated subjects depending on the date they were accepted for the MBCT course (before or after 8 weeks until onset of the intervention) and Kingston et al. (2007) on the day of referral, and therefore they present a high risk of bias. Only one out of the remaining studies did not report the randomization method. Eight out of the 11 studies adequately concealed the allocation and five assessed outcomes blinded to patients’ allocation. Four studies showed an attrition >20% and four used intention-to-treat analyses. Most studies did not report fidelity of treatment implementation or investigated potential contamination between groups.

In this review we aimed to understand how mindfulness/acceptance therapies affect ruminative thoughts in patients with current or past depressive disorders. Eleven studies were identified in the literature, nine RCT of good methodological quality and two pRCT. There was some heterogeneity among them (e.g. patients in diverse phases of their depressive disorder, number of past episodes, scales used, different attrition rates), but results were statistically consistent. They show that MBCT, compared to usual care, produces a significant and moderate (according to the usual parameters, Cohen, 1988) reduction in rumination. This effect seems independent of the treatment phase (acute or maintenance) or the number of past depressive episodes, and it was maintained one month after the end of treatment. On the other hand, mindfulness/acceptance procedures seems as effective at reducing rumination as other treatments such as medication, CBT and behavioral activation. However, these results must be interpreted with caution given the low number of studies included, but they are in line with the narrative review published by Barraca (2012), who concluded that acceptance and mindfulness strategies are as or more effective than others such as focalized distraction or some forms of restructuring. However, it can not be ruled out that part of the explanation of this outcome has to do with the common factors of the different therapies that are used for the treatment of depression.

A first consideration is whether these results could be considered unexpected, since mindfulness/acceptance process do not try to reduce (or suppress) unpleasant thoughts, but the emotional impact they produce on the individual. Indeed, some laboratory studies with healthy participants have shown that mindfulness/acceptance strategies, compared to suppression techniques, did not reduced intrusive thoughts but the stress associated with them (Marcks & Woods, 2005; Najmi, Riemann, & Wegner, 2009). Unfortunately, the rumination scales used in the included studies assess the occurrence of intrusive thoughts but not their emotional impact. It is possible that the reduction of ruminative thinking is related to the fact of not “feeding” those thoughts, as it has been considered that the most frequent onset and the highest intensity of certain thoughts could be related to trying to avoid or reject them, or make them disappear, in accordance with the theory of the ironic process of mental control (Wegner, 1994). Therefore, although the purpose of acceptance and mindfulness procedures was to allow the ruminative or intrusive thoughts to occur, albeit affecting the patient to a lesser extent, in the end, paradoxically, they would reduce it because of the thoughts not being stimulated by means of attempts to control, restructure or reduce them. This is a desirable clinical outcome, as it would lead to improvement in the patient, but at the current state of research it is not possible to conclude whether improvements in rumination scores are due to the patients’ altered relationship with their ruminative thoughts or to the frequency/intensity of these.

A second consideration refers to whether the effects observed on rumination are actually produced by the proposed psychological mechanisms (mindfulness/acceptance). Although most of the included studies applied measures of the participants’ perception of their own mindfulness/acceptance skills, only four reported the associations of these measures with rumination (using different statistical approaches). At the correlational level, Van Aalderen et al. (2012) found a statistically significant negative correlation (r=−.56, p<.001) among the scores on the subscale “accept without judgment” (from the Kentucky Inventory of Mindfulness Questionnaire, KIMS) and the level of rumination, whereas Bieling et al. (2012) did not observe a significant association using the Toronto Mindfulness Scale. Batink et al. (2013) found that changes produced in rumination by the MBCT intervention were significantly mediated by KIMS scores. Finally, Manicavasagar et al. (2012) observed that post-scores in the Mindful Attention Awareness Scale (MAAS) significantly related to lower rumination in the MBCT group, but not in the CBT condition. This is an interesting result since MBCT did not outperform CBT in the increasing of mindfulness skills or the reduction of rumination; this suggests that MBCT training, rather than increasing mindfulness skills compared to CBT, could enable patients to orientate the application of those skills to their ruminative thinking, whereas CBT would decrease rumination through other psychological mechanisms.

Third, ruminative thoughts have been found as a central mechanism in the onset and maintenance of depressive functioning (Gotlib & Joormann, 2010; Martell et al., 2001), and therefore we analyzed the mediator role of rumination in the effect of interventions on depressive symptoms. The five studies providing data obtained mixed results, and the heterogeneity of the statistical procedures applied precluded a quantitative synthesis of the data reported. Instead, we performed an exploratory subgroup meta-analysis that showed a better antidepressant effect in the studies that obtained a stronger effect on rumination. The only study that compared MBCT to CBT, Manicavasagar et al. (2012), found that rumination significantly predicted symptoms at post-intervention (when treatment were decreasing both depression symptoms and ruminative thoughts), and it could raise the question of whether rumination is a central process to feel depressed or it is part of the core of symptoms of depression. In this last case, the decrement of ruminative thoughts can be understood as part of therapy efficacy.

From a clinical perspective, mindfulness/acceptance interventions have shown to be as effective to reduce depressive symptomatology as antidepressant drugs or CBT (Bockting, Hollon, Jarrett, Kuyken, & Dobson, 2015; Clarke, Mayo-Wilson, Kenny, & Pilling, 2015; Khoury et al., 2013), but the evidence on their mechanisms of action is still scarce. In this review we have presented an integrated view of the empirical findings about the effect of mindfulness/acceptance interventions on ruminative thoughts in depression. Results are positive and furthermore suggest that changes in rumination are mediated by mindfulness/acceptance processes, and that they in turn mediate in the clinical effects of interventions.

However, these conclusions are constrained by several limitations, such as the few studies identified, the heterogeneity of the measures used, the lacking of reporting about associations between rumination, self-perceived mindfulness skills and depressive symptoms, or the scarcity of follow up data. There is a need of more well controlled studies with real patients, that compare the most commonly used cognitive-behavioral techniques to treat ruminative and intrusive thoughts (e.g. restructuring, cognitive exposure or stopping the thought) to the acceptance and mindfulness techniques. It also should be necessary to carry out a comparison of the psychometric properties of the existing instruments to assess rumination, as well as including measures of the participants’ perception of their mindfulness/acceptance skills, and of the impact of ruminative thoughts on their psychological functioning. Assessments of these variables should be taken at different time-points during therapy, to analyze the temporal evolution of the changes produced by the intervention (i.e. whether the acceptance of ruminative thoughts occurs before their reduction or not). Medium and long-term follow up data are needed, since relapse and recurrence are common in major depression and different therapeutic strategies could lead to different long-term effects. A more sound knowledge of these aspects will enable a more efficient application of psychological therapies to treat depression, as well as a more personalized treatment according to the different patients’ psychological traits and skills. But along with this type of approach, the analysis from a more experiential perspective of the experience of rumination on the part of patients will be another of the challenges for future research.