This is Joel Guarna' s conclusion: " ACT and CBT as a larger tradition are becoming more difficult to distinguish over time"- see last paragraphs.

Moderators and non-specific predictors of treatment outcome for anxiety disorders: a comparison of cognitive behavioral therapy to acceptance and commitment therapy.

Wolitzky-Taylor KB, Arch JJ, Rosenfield D, Craske MG.J Consult Clin Psychol. 2012 Oct;80(5):786-99. doi: 10.1037/a0029418. Epub 2012 Jul 23.PMID: 22823858 Clinical Trial.

therapy, cognitive behavioral therapy, and acceptance and commitment therapy in the treatment of adolescents

Article

• Jul 2014 Researchgate
• Figure CaptionTable 1. Comparison between CBT, ACT, DBT, and MDT… Read moreThis figure was uploaded by Joan SwartContent may be subject to copyright.

Free PMC articledoi: 10.1037/a0028310. Epub 2012 May 7.Randomized Controlled Trial. 2012 Oct;80(5):750-65.J Consult Clin PsycholRandomized clinical trial of cognitive behavioral therapy (CBT) versus acceptance and commitment therapy (ACT) for mixed anxiety disorders Joanna J Arch 1, Georg H Eifert, Carolyn Davies, Jennifer C Plumb Vilardaga, Raphael D Rose, Michelle G CraskeAffiliations expandPMID: 22563639 PMCID: PMC4718567 DOI: 10.1037/a0028310

Abstract

Objective: Randomized comparisons of acceptance-based treatments with traditional cognitive behavioral therapy (CBT) for anxiety disorders are lacking. To address this gap, we compared acceptance and commitment therapy (ACT) to CBT for heterogeneous anxiety disorders.

Method: One hundred twenty-eight individuals (52% female, mean age = 38, 33% minority) with 1 or more DSM-IV anxiety disorders began treatment following randomization to CBT or ACT; both treatments included behavioral exposure. Assessments at pre-treatment, post-treatment, and 6- and 12-month follow-up measured anxiety-specific (principal disorder Clinical Severity Ratings [CSRs], Anxiety Sensitivity Index, Penn State Worry Questionnaire, Fear Questionnaire avoidance) and non-anxiety-specific (Quality of Life Index [QOLI], Acceptance and Action Questionnaire-16 [AAQ]) outcomes. Treatment adherence, therapist competency ratings, treatment credibility, and co-occurring mood and anxiety disorders were investigated.

Results: CBT and ACT improved similarly across all outcomes from pre- to post-treatment. During follow-up, ACT showed steeper linear CSR improvements than CBT (p < .05, d = 1.26), and at 12-month follow-up, ACT showed lower CSRs than CBT among completers (p < .05, d = 1.10). At 12-month follow-up, ACT reported higher AAQ than CBT (p = .08, d = 0.42; completers: p < .05, d = 0.56), whereas CBT reported higher QOLI than ACT (p < .05, d = 0.42). Attrition and comorbidity improvements were similar; ACT used more non-study psychotherapy at 6-month follow-up. Therapist adherence and competency were good; treatment credibility was higher in CBT.

Conclusions: Overall improvement was similar between ACT and CBT, indicating that ACT is a highly viable treatment for anxiety disorders.

(Nice explanation if you wish: Comparing ACT and CBT

Submitted by Joel Guarna

From my website, www.whitepinepsych.com:

Treatment Approaches

Cognitive-Behavioral Therapy What is Cognitive-Behavioral Therapy (CBT)? The history of CBT dates back to the seminal work of B.F. Skinner, the father of modern behavior therapy. At that time, behavior therapy was a reaction to the traditional Freudian forms of psychotherapy that were only loosely based on scientific principles and were difficult to subject to rigorous scientific study. Skinner held psychology accountable as a science of human behavior, forever changing the face of psychotherapy. Techniques drawn from Skinner's basic behavioral science continue to be employed with good effect in modern psychotherapy. Skinner's account, however, had its limitations. The most notable limitation was that his account of human language and cognition failed to generate a vigorous line of basic research, limiting its evolution to forms that could be employed with patients with complicated psychological problems. Instead, the field opened to the work of Albert Ellis and Aaron T. Beck, the founders of modern cognitive therapy. Ellis and Beck, and their many successors, transformed the practice of psychotherapy by emphasizing therapy techniques that aimed to change the content and manner of one's thinking, not just their overt behavior. Cognitive therapy and behavior therapy continued to cross-fertilize each other over the past several decades. Modern CBT incorporates both cognitive and behavioral techniques. CBT has become the most well-known, mainstream approach to therapy, partly because it has, by far, the strongest research support for its effectiveness in treating a wide range of emotional and behavioral problems. CBT has been found effective in treating depression, anxiety disorders, the effects of trauma, substance abuse and addiction, complications related to medical conditions, and many other conditions.

Acceptance and Commitment Therapy What is Acceptance and Commitment Therapy (ACT, said as the word "act")? ACT, just approaching its 30th anniversary since its inception, is an innovative form of behavioral and cognitive therapy that has built upon both the strengths and the weaknesses of traditional cognitive-behavioral therapy (CBT). ACT is based on a behavioral account of human language and cognition called Relational Frame Theory (RFT), which has "filled in the holes" left by Skinner's theories. RFT, in contrast to Skinner's accounts, has generated a vigorous body of basic research into human language and cognition, providing fuel for the development of new treatment approaches. The "fruit" of this progress can be found in the philosophy and basic concepts underlying ACT. ACT has moved away from the traditional CBT emphasis on changing or correcting one's thoughts in order to alleviate suffering. Instead, ACT aims to alter the functions of our private experiences (thoughts, feelings, memories, bodily reactions), so they no longer entangle us. Said another way, ACT aims to change our relationship with these private events so we can become free from their grip, and free from the patterns that bind us and prevent us from living a flexible, meaningful, and enjoyable life. In the service of these aims, ACT incorporates acceptance strategies, mindfulness techniques, and a wide range of behavioral approaches already known to be effective from CBT. ACT is one of a family of interventions inside the CBT tradition writ large that are focusing on the person's relationship to experiences rather than on the content of these experiences. The data on ACT and related approaches are moving CBT itself toward a new model that emphasizes being open, centered, mindful and actively pursuing values. Because of that, ACT and CBT as a larger tradition are becoming more difficult to distinguish over time.)

Some research has started to be conducted on relative effectiveness. Some indicate one versus the other as being more effective. It likely depends on the context in which you are comparing the interventions. I attached some references and abstract quotations below:

Hancock, K. M., Swain, J., Hainsworth, C. J., Dixon, A. L., Koo, S., & Munro, K. (2018). Acceptance and commitment therapy versus cognitive behavior therapy for children with anxiety: Outcomes of a randomized controlled trial. Journal of Clinical Child & Adolescent Psychology, 47(2), 296-311.

From their abstract:

"There was some support for ACT having greater effect sizes for QOL outcomes but not for avoidance/fusion. Although this study does not suggest that ACT is equivalent to CBT or should be adopted in its place, it does provide evidence that ACT might be another empirically supported treatment option for anxious youth. Further research is needed to replicate these findings."

Barrett, K., & Stewart, I. (2021). A preliminary comparison of the efficacy of online Acceptance and Commitment Therapy (ACT) and Cognitive Behavioural Therapy (CBT) stress management interventions for social and healthcare workers. Health & Social Care in the Community, 29(1), 113-126.

From their abstract:

"Outcomes indicated that both interventions resulted in significant improvements in stress, burnout and mental health scores from baseline to post‐treatment. No significant differences were observed between ACT and CBT conditions, or in psychological flexibility scores from baseline to post‐treatment"

Khayatan, S., Aghaei, A., Abedi, M., & Golparvar, M. (2020). Comparison of CFT-ACT Combined Therapy with CBT on Psychological Well-Being in Diabetic Women. Iranian journal of diabetes and obesity, 12(2), 69-75.

From their abstract: "The results showed that the scores of experimental groups participants in the post test of compassion therapy-ACT and CBT improved significantly compared to the control group in self-acceptance, positive relationships with others, purposeful life, individual development, environmental dominance and autonomy subscales (P-value= 0.001) .There exist a significant difference  between the experimental group of compassion focused- ACT combined therapy and CBT in self-acceptance, positive relationships with others, purposeful life and individual -development  subscales (P-value< 0.001).

This is a complex question and is not easily answered as a dichotmoty However, I follow the leanings of the great but recently-deceased Scott Lilienfeld on this topic. Below is a link to a piece in which he criticizes ACT as being a lot of hype with less than rigorous evidence to support it. I hope you find it useful.

Cory

https://link.springer.com/content/pdf/10.1007/s10879-015-9319-y.pdf

Normally, the best and with the greatest and best scientific evidence is COGNITIVE BEHAVIORAL THERAPY (CBT o TCC)