INTRODUCTION
Major depressive disorder (MDD) is a prevalent and debilitating psychiatric disorder characterized by a variety of affective, cognitive, and somatic symptoms. The MDD showed high lifetime prevalence, follows chronic course, and is associated with increased mortality [1,2]. Consequently, it is a major contributor for global burden of mental illness [3].
Pharmacotherapy and psychotherapy are the two major therapeutic approaches for MDD. Although pharmacotherapy offers several advantages, such as accessibility and fast responsiveness, many patients—particularly those with chronic courses—do not achieve sufficient improvement with medication alone [4]. Additionally pharmacotherapy may be limited by side effects and treatment discontinuation [5]. Therefore, the importance of psychological therapy has been equally emphasized in the treatment of MDD. Among psychological approaches, cognitive-behavioral therapy (CBT) is the most widely used and extensively studied treatment, and it is recommended in treatment guidelines [6].
Recently, the “third wave” of CBT has emerged, representing process-based, contextual treatments that focus on the function of thoughts and actions rather than their specific contents [7]. Among these approaches, acceptance and commitment therapy (ACT) has gained prominence with a conceptual framework aimed at increasing transdiagnostic psychological flexibility [8,9]. ACT is grounded in relational frame theory, which integrates human language with internal experiences, such as thoughts and feelings, that contribute to cognitive fusion. While this type of learning is a normal human function, excessive cognitive fusion can lead to experiential avoidance, where individuals attempt to evade anticipated unwanted experiences [8,9]. Cognitive fusion and experiential avoidance may perpetuate depressive symptoms through mechanisms such as rumination and behavioral inactivation.
Several studies have demonstrated the effectiveness of ACT in treating depressive disorders. ACT has been applied in various formats, including group and individual settings, and online applications have also been utilized in several studies [10]. Recent meta-analyses have shown that ACT is effective in reducing depressive symptoms and maintaining these improvements over time [10,11]. However, only a few studies have specifically examined its effectiveness for clinically diagnosed MDD, and these studies were conducted in individual settings [12,13]. Furthermore, study sites have been relatively limited; for instance, in East Asia, only China reported results from randomized controlled trials (RCT) [10]. Therefore, further research is needed to evaluate the effectiveness of ACT for MDD across diverse settings and regions to enhance the generalizability of previous findings.
In this study, we developed a group-based ACT (GACT) program for MDD based on our previous protocol for depression and designed a RCT to evaluate the effectiveness of this program. The aims of this study are as follows: first, to investigate the effects of GACT on the treatment of patients with MDD in Korea; and second, to examine which process changes are associated with reductions in depressive symptoms using various process measures.
METHODS
Study design
This study was a single-blind RCT conducted at a single center, with patients randomly assigned in a 1:1 ratio to either the GACT or waitlist control (WLC) group. After allocation, therapists provided therapy to the GACT group, while the WLC group received GACT after the wait-list period. The study was approved by the Institutional Review Board of Kyungpook National University Hospital (2021-04-032).
Participants
Patients with MDD, aged 18-40 years, were recruited through subway advertisements and psychiatric clinics at Kyungpook National University Hospital. The psychiatrist (L.S.J) conducted interviews and made diagnoses using the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, 5th Edition Disorders-Clinical Version. Patients with acute medical or neurological disorders, intellectual disabilities, a history of brain trauma, or other major psychiatric illnesses, including schizophrenia, were excluded. Following screening via phone interviews, 72 participants were invited for in-person interviews, of whom 13 were excluded based on the study’s exclusion criteria. After providing written informed consent, 59 participants were randomly assigned to either the GACT or WLC group using a computer-generated randomization sequence with block sizes of four. This was a single-blind design in which participants remained unaware of their group assignment throughout the 8-week trial. While participants in both the GACT and WLC groups were allowed to continue their usual pharmacotherapy, other structured psychological treatments, such as traditional CBT, or neurostimulations, such as transcranial magnetic stimulation, were not allowed during the 8-week study period. All 50 participants who completed our research protocol (attending at least six sessions and completing both pre- and post-assessments) were included in the analysis (GACT, n=27; WLC, n=23) (Figure 1).
Components of group acceptance and commitment therapy for MDD
Based on the previous protocols for depression [14-16], a group-format ACT program consisting of 8 sessions was developed. Each GACT group consisted of 3-5 participants and was offered free of charge. Session 1 introduced the symptoms of MDD and explained how ACT components relate to the initiation and maintenance of MDD. For example, distress intolerance was highlighted as an important factor contributing to experiential avoidance, which, in turn, maintains MDD symptoms. Session 2 focused on the concept of creative hopelessness and explored the relationship among rumination, cognitive fusion and experiential avoidance. Diaphragmatic breathing training was conducted with therapists. In Sessions 3 and 4, cognitive defusion techniques were introduced and practiced. The concept and importance of mindfulness were also explained. From Sessions 3 to 7 incorporated mindfulness training, practiced alongside diaphragmatic breathing in a seated position. Session 5 emphasized how increased willingness can facilitate the acceptance of uncomfortable feelings, thereby encouraging greater participation in activities. Session 6 and 7 highlighted the importance of personal values in one’s life and guided participants identifying their own values. Participants were also encouraged to set goals based on these values and engage in them with self-compassion. Session 8 summarized all the previous sessions and covered relapse prevention. A detailed description of the GACT protocol is provided in Supplementary Table 1.
Therapists
Two psychiatrists and one clinical psychologist participated as therapists. The corresponding author, an experienced therapist in the field of CBT, trained the other therapists in the GACT program before its implementation. Additionally, the corresponding author supervised the overall deployment of the program. The other two therapists had over 10 years of clinical experience in psychiatry and psychology. The outcome of a GACT program for obsessive-compulsive disorder (OCD), conducted by the same therapists, have already been published [17].
Measures
Primary outcome measures
The Center for Epidemiological Studies-Depression Scale (CES-D) is a 20-item self-report scale used to measure the current severity of symptoms of depression [18]. This comprehensive assessment covers various symptom domains, including affective, cognitive, and somatic aspects. Scores range between 0 to 60, with well-established internal consistency and validity. In this study, the Korean version of the CES-D was used (Cronbach’s α=0.91) [19].
Secondary outcome measures
Secondary outcome measures were selected to evaluate the processes enhanced by GACT treatment. The Ruminative Response Scale-Revised (RRS-R), consisting of 10 items, was used to measure maladaptive self-reflective thinking, referred to as rumination, and was validated with good internal consistency [20]. Each item was rated on a four-point Likert scale ranging from 1 (never) to 4 (always), with a higher score indicating a higher degree of rumination. In this study, we used the Korean version of the RRS-R (Cronbach’s α=0.89) [21]. The Thought Action Fusion Scale (TAFS) [22] consists of 19 items that measure the degree of thought-action fusion belief and is divided into three categories: TAF-morality (12 items), TAF-likelihood-self (3 items), and TAF-likelihood-others (4 items). Each item is rated on a 5-point scale ranging from 0 (strongly disagree) to 4 (strongly agree). We used the recently validated Korean version of the TAFS, which had good internal consistency (Cronbach’s α=0.92) [23]. The Acceptance and Action Questionnaire-II (AAQ-II) [24] is a tool for evaluating the psychological inflexibility and experiential avoidance, key concepts emphasized in ACT treatment. The Korean version of the AAQ-II, which contains eight items rated on a 7-point Likert scale, was used, with higher scores indicating a higher degree of psychological inflexibility. This version had good internal consistency (Cronbach’s α=0.85) [25]. The Cognitive Fusion Questionnaire (CFQ) [26] is a 7-item questionnaire developed to measure the tendency toward excessive attachment to the literal content of thoughts. Each item is rated on a 7-point Likert scale, with higher scores indicating higher levels of cognitive fusion. This study used the validated Korean version of the CFQ, which demonstrated good internal consistency (Cronbach’s α=0.91) [27].
In addition to the process measures, the Generalized Anxiety Disorder-7 (GAD-7) was included to assess anxiety symptoms commonly co-occurring with depressive symptoms. The GAD-7 is a 7-item scale rated on a 4-point Likert scale (0-3), with higher scores indicating greater anxiety severity [28]. A validated Korean version was used in this study [29].
Statistical analysis
Group differences in demographic characteristics, including age, sex, education level, and primary and secondary outcome measures at baseline were evaluated using t-tests for continuous variables and chi-squared tests for categorical variables. Group effects on changes in measures over time (pre-treatment and post-treatment) were assessed using linear mixed-effects models. In this analysis, group, time, and the interaction between time and group were included as fixed effects, with participants included as random intercepts to account for repeated measures. Post-hoc pairwise comparisons were conducted to calculate between- and within-group effect sizes, which were expressed as Cohen’s d, using independent t-tests for between-group differences and paired t-tests for within-group differences. Furthermore, to clarify process measures related to the efficacy of MDD treatment, correlation and multiple regression analyses were used to evaluate the relationship between changes in CES-D scores and secondary outcome measures. All data analyses were performed using Jeffrey’s Amazing Statistics Program (https://jasp-stats.org).
RESULTS
Sample characteristics
The baseline demographic and clinical characteristics of the selected participants are presented in Table 1. The mean (±standard deviation) ages of the GACT and WLC groups were 26.8±5.2 and 26.6±5.5 years, respectively. Based on the CES-D scores, patients assigned to the GACT group exhibited symptom severity ranging from moderate to severe. No group differences were observed in terms of age, sex, and education. Additionally, no group differences were also observed in our primary and secondary outcome measures.
Among the 50 participants, 15 (30%) were drug-naïve or not currently taking any medication. There were no significant between-group differences in the proportion of participants on medication at baseline (χ2=0.56, p=0.76). Participants were instructed to maintain their existing medication regimens throughout the study when feasible; however, three individuals in the GACT group and one in the WLC group discontinued their medication during the study period. Regarding antidepressants, five participants in the GACT group and one in the WLC group reduced their dosage, while two participants in the GACT group and three in the WLC group increased their dosage. Detailed information about the baseline medications is provided in Table 2.
Primary outcome measure
Linear mixed-effects model analyses revealed significant differences between the GACT and WLC groups in the CES-D scores from pre- to post-treatment. Post-hoc comparisons showed that compared to the WLC group, the GACT group exhibited significant improvements in CES-D at post-treatment (d=-1.46). Within the GACT group, CES-D scores were reduced between the pre- and post-treatment points, with a large effect size (d=-1.77) (Table 3 and Figure 2).
Secondary outcome measures
The results of the process measures before and after treatment are presented in Table 4. In the linear mixed-effects model analyses, significant group-by-time interactions were observed in the RRS-R, TAFS, AAQ-II, CFQ, and GAD-7 scores. Post-hoc pairwise comparisons showed that, compared to the WLC group, the GACT group exhibited significant improvements in all secondary outcome measures (d=-0.77 to -1.14). Within the GACT group, all measurements decreased post-treatment compared to pre-treatment (d=-0.68 to -1.79).
Normality was evaluated using the Shapiro-Wilk test. All variables from the pre- and post-assessments met the normality assumption (p>0.05), except for the post-treatment scores of the TAFS and GAD-7 in the GACT group (p=0.002 and p<0.001, respectively). However, the distribution of the difference scores (post-pre) for the TAF and GAD-7 did not violate the normality assumption (p>0.05).
Relationship between changes in depressive symptoms and process measures
Pearson correlation analyses revealed that changes in the CES-D scores were significantly correlated with reductions in the scores of AAQ-II (r=0.56, p=0.003) and CFQ (r=0.38, p=0.048) (Table 5 and Figure 2). An additional multiple regression analysis showed that changes in AAQ-II scores were significantly associated with changes in MDD symptoms, after controlling for the effects of other process measures (Table 6).
DISCUSSION
This study provides evidence supporting the efficacy of GACT in treating individuals diagnosed with MDD. Participants who received GACT exhibited a significant reduction in depressive symptoms, with an average decrease of 60.6% from baseline to the 8-week follow-up as measured by CES-D. In contrast, the WLC group showed minimal change over the same period. While previous studies suggest favorable effects on depression similar to our results [30], few randomized controlled studies, especially those using a group-format ACT, have been conducted for MDD patients. In this regard, these findings may extend the usefulness of ACT by exploring its therapeutic impact within a broader context and among Korean population.
Our GACT program effectively reduced depressive symptoms, consistent with findings from recent meta-analyses [10,11]. Compared to similar randomized controlled studies using a group-based, face-to-face program, our results align with previous findings. For instance, in patients with MDD, a 6-week (12-session) GACT program resulted in a 15.3% symptom reduction, which increased to 37.9% at a 3-month follow-up [31]. These findings support GACT as an effective intervention for reducing depressive symptoms in MDD patients. Additionally, our GACT program effectively reduces rumination, one of the most significant maladaptive cognitive patterns in MDD [32]. Rumination is linked to cognitive fusion and experiential avoidance, both contributing to psychological inflexibility [33]. ACT helps patients disengage from repetitive negative thinking, significantly reducing rumination and improving depressive symptoms [34]. A previous study also found that ACT was effective in reducing both depression and rumination [35]. Therefore, GACT is a valuable tool for improving depressive symptoms and addressing underlying maladaptive cognitive patterns.
The measurement of core ACT processes—AAQ-II, CFQ, and TAFS—demonstrated that the GACT group showed significant improvements compared to the WLC group in this study. These findings suggest that the elements of ACT, such as experiential avoidance and cognitive fusion, are associated with changes in depressive symptoms. More specifically, we observed that changes in CES-D scores were strongly correlated with AAQ-II scores, but not with TAF. This suggests that, from the ACT perspective, experiential avoidance may be the primary psychopathology of depression, and that reducing it has a substantial impact on alleviating depressive symptoms. According to ACT theory, an individual’s attempts to avoid negative internal experiences—such as distressing emotions, thoughts, or memories—can provide temporary relief, reinforcing these avoidance behaviors and ultimately resulting in and sustaining various depressive symptoms [36-38]. In fact, previous studies have consistently documented a robust association between experiential avoidance and depressive symptoms in individuals with depression [39,40]. Therefore, ACT holds a critical position in psychological treatment of depression, offering a unique therapeutic process that fosters acceptance and willingness, which can reduce experiential avoidance [41].
Moreover, it is noteworthy that similar findings have been observed in other disorders that are presumed to share common therapeutic factors. Research on the efficacy of ACT in anxiety disorders and posttraumatic stress disorder supports its broader applicability [39,42,43]. In particular, anxiety disorders show high comorbidity with MDD and exhibit characteristics that share transdiagnostic similarities [44]. For instance, AAQ-II scores showed a positive correlation with CES-D scores in both OCD patients and healthy controls [45]. Consistent with current observations in MDD, improvements in experiential avoidance, cognitive fusion, and thought-action fusion were also observed in OCD following a similar 8-week group-based ACT treatment. This convergence of treatment mechanisms supports a transdiagnostic approach to intervention, wherein targeting these shared processes may yield beneficial outcomes across a range of psychopathologies. However, our previous research also identified cognitive fusion as a major contributor to OCD treatment outcomes [17]. The finding contrasts with the one showing that experiential avoidance, as measured by the AAQ-II, was the significant predictor of MDD treatment outcome. Taken together, the current and previous studies suggest both shared and distinct factors across psychiatric illnesses, highlighting the need to refine these factors for more effective treatments.
Recent meta-analyses of 23 RCTs suggest that, although face-to-face individual ACT showed the largest effect size in reducing depressive symptoms, there were no significant differences in treatment outcomes or acceptability (as indicated by dropout rates) between individual and group ACT formats [46]. The effect sizes, measured by standardized mean differences in the reduction of depressive symptoms compared to control, were 1.44 for individual ACT and 1.34 for group ACT [46], which is consistent with our findings. The group format offers additional advantages, including cost-effectiveness [47] and the facilitation of therapeutic change through group dynamics [48], supporting group ACT as a practical alternative to individual therapy. Furthermore, unlike traditional CBT, which targets the modification of maladaptive thought content and beliefs, ACT emphasizes enhancing psychological flexibility by promoting acceptance of distressing experiences and detachment from negative thoughts. Although the two treatments target different processes, a head-to-head RCT reported substantial symptom reductions in both ACT and CBT, with no significant difference in effectiveness [12]. Therefore, our GACT program may serve as an effective alternative treatment for MDD.
This research had some limitations. First, the relatively small sample size and single-center design may limit the statistical power and generalizability of our findings. Second, among our participants, 29 individuals (58%) exhibited moderate or higher levels of anxiety symptoms (GAD-7 score ≥10). MDD is frequently comorbid with anxiety symptoms, and our participants showed similar patterns, consistent with previous studies [49,50]. Although it is possible that our ACT program exerts transdiagnostic effects on both depressive and anxiety symptoms, its specific effects on MDD are difficult to determine. Therefore, a more refined study design is needed to minimize the influence of comorbid conditions such as generalized anxiety disorder. Third, the participants were confined to a narrow age range (18-40 years), which restricts the applicability of the results to older adults or other demographic groups. Fourth, all participants continued to receive pharmacotherapy during the study, potentially confounding effects. However, in this study, there were no significant group differences in the proportion of participants on medication at baseline. Fifth, the use of a WLC group, rather than an active control condition, may have introduced expectancy effects that could influence treatment outcomes. Sixth, reliance on self-report measures for primary and secondary outcomes raises the possibility of response biases. While our study employed a group-based approach, it was difficult to schedule additional individual meetings to assess symptoms for each participant. Nevertheless, the inclusion of clinician-rated scales may enhance the objectivity of outcome assessments in future research. Finally, to confirm the long-term treatment effect, future research should extend the follow-up period.
In conclusion, this study suggests that GACT significantly reduces depressive symptoms and enhances psychological flexibility in Korean patients with MDD. Furthermore, reductions in experiential avoidance were strongly correlated with improvements in depressive symptoms, thereby supporting the efficacy of GACT as a promising therapeutic modality.
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