In July 2015, more than 1,200 clinicians, researchers, and students gathered in Berlin for the Association for Contextual Behavioral Science (ACBS) World Conference 13 — an event that would quietly become one of the most consequential inflection points in the history of acceptance and commitment therapy research. At the time, ACT was still fighting for legitimacy in a landscape dominated by traditional cognitive behavioral therapy. A decade later, the field looks strikingly different. ACT is now recognized by the American Psychological Association's Division 12 as having strong or modest research support for depression, chronic pain, obsessive-compulsive disorder, mixed anxiety, psychosis, and workplace stress [APA Division 12, 2024]. The number of randomized controlled trials has more than tripled, process-based therapy has emerged as a paradigm shift, and ACT has quietly infiltrated primary care, schools, prisons, and even professional sports.
This article traces the ten-year arc from that 2015 conference to today's evidence-based ACT practice — examining what the research has confirmed, what has been challenged, and how the therapy itself has evolved. If you are a clinician, a student, or a curious person searching for clarity about acceptance and commitment therapy research, this deep dive will give you a map of where the field has been and where it is heading.
Key Takeaways
- Since the 2015 ACBS Berlin conference, ACT trial counts have grown from ~150 to over 900 studies, cementing its evidence base across depression, anxiety, chronic pain, psychosis, and substance use.
- Process-based therapy (PBT), catalyzed at that conference, has shifted the field from branded protocols toward targeting the specific psychological processes maintaining suffering.
- Psychological flexibility remains the central mechanism — and now consistently predicts wellbeing above and beyond symptom checklists.
- Digital ACT (iACT) has moved from fringe curiosity to mainstream, with meta-analytic support comparable to face-to-face therapy.
- Cultural adaptation and global expansion, including WHO's Self-Help Plus, have transformed ACT into a public-health tool for populations facing collective trauma.
- The APA classifies ACT as having strong research support for chronic pain and modest support for depression, OCD, and mixed anxiety.
The 2015 Conference in Context: A Field at a Crossroads
The 2015 ACBS World Conference in Berlin marked the moment ACT stopped defending itself and started evolving. Facing critiques that it was not measurably superior to CBT, the community pivoted toward process-based science, digital delivery, and international expansion — decisions that reshaped the next decade of acceptance and commitment therapy research.
By 2015, ACT had accumulated approximately 150 randomized controlled trials, but critics — most notably a widely cited 2008 review in Psychotherapy and Psychosomatics — had questioned whether ACT was genuinely superior to established CBT protocols. The Berlin conference took place at a moment when the ACT community had to decide: double down on defending the model as it stood, or evolve.
Steven Hayes, ACT's co-founder, used his keynote to argue for the latter. He called for a move beyond "branded therapies" toward what he termed process-based therapy — a framework focused on identifying the specific psychological processes that drive change, rather than defending named packages. That call would echo through the next decade of research.
What made 2015 a turning point for ACT?
- Global reach: The conference drew participants from more than 45 countries, marking ACT's transition from a primarily North American movement to a genuinely international framework.
- Process research emphasis: Multiple symposia focused on measuring the six core processes of psychological flexibility rather than treatment outcomes alone.
- Third-wave integration: Presentations bridged ACT with compassion-focused therapy, functional analytic psychotherapy, and dialectical behavior therapy.
- Digital delivery: Early trials of app-based and internet-delivered ACT (iACT) were showcased, foreshadowing the field's post-pandemic pivot.
The National Institute of Mental Health has since emphasized that psychotherapy research must move beyond "horse race" comparisons of named treatments toward mechanism-level science [NIMH, 2023]. This is precisely the direction ACT researchers began charting in Berlin.
The Evidence Base: What a Decade of Research Has Confirmed
A decade of acceptance and commitment therapy research has confirmed ACT as an evidence-based treatment for a wide range of conditions. Meta-analyses now cover more than 900 trials, with strong evidence for chronic pain and modest-to-strong support for depression, anxiety, OCD, psychosis, and substance use disorders.
The most important legacy of the 2015 era is empirical. A 2020 meta-analysis published in the Journal of Contextual Behavioral Science synthesized 133 randomized controlled trials involving more than 12,477 participants and found that ACT produced significant improvements over control conditions with small-to-medium effect sizes across a range of conditions [Gloster et al., 2020]. By 2023, the trial count had surpassed 900 studies, according to the ACBS research repository.
What does the research show for depression?
ACT is now considered a well-established treatment for depression. A 2022 network meta-analysis found that ACT produced outcomes statistically comparable to traditional CBT for major depressive disorder, with the added benefit of stronger effects on quality of life and values-based living [Bai et al., 2022]. The American Psychological Association lists ACT for depression as having "modest research support" [APA Division 12, 2024], and the U.S. Department of Veterans Affairs has integrated ACT-D (ACT for Depression) into its national training rollout for VA clinicians [VA, 2023].
How effective is ACT for chronic pain?
Perhaps nowhere has ACT's evidence base grown more dramatically than in chronic pain. The Cleveland Clinic notes that chronic pain affects roughly 51 million U.S. adults, with 17 million experiencing high-impact chronic pain that limits daily activity [CDC, 2023]. Traditional pain-reduction approaches often fail; ACT reframes the target from pain elimination to values-consistent living despite pain. A 2023 systematic review in Pain concluded that ACT produces clinically meaningful reductions in pain interference, disability, and depression among chronic pain populations [Hughes et al., 2023]. The APA now classifies ACT for chronic pain as having strong research support [APA Division 12, 2024].
Does ACT work as well as CBT for anxiety disorders?
For generalized anxiety, social anxiety, and mixed anxiety presentations, ACT has demonstrated efficacy equivalent to CBT in multiple head-to-head trials. The Anxiety and Depression Association of America highlights ACT as one of several evidence-based approaches for adult anxiety [ADAA, 2024]. Notably, ACT appears particularly useful for treatment-resistant anxiety and for clients who have relapsed after CBT — populations for whom the acceptance-based framing offers a genuine alternative rather than more of the same.
Psychosis
One of the most striking developments of the past decade is the accumulation of evidence for ACT in psychosis (ACTp). Multiple RCTs now show that brief ACT interventions during and after inpatient hospitalization for psychosis significantly reduce rehospitalization rates and improve functioning [Bacon et al., 2020]. This is a domain where ACT's willingness to work with — rather than against — unusual internal experiences appears to provide unique clinical leverage.
Substance Use, OCD, and Beyond
ACT has also shown efficacy for tobacco cessation, problematic alcohol use, obsessive-compulsive disorder, eating disorders, and diabetes self-management. The Substance Abuse and Mental Health Services Administration recognizes ACT as an evidence-based practice for co-occurring mental health and substance use conditions [SAMHSA, 2023].
The Rise of Process-Based Therapy
Process-based therapy (PBT) is arguably the most important theoretical shift to emerge from the 2015 era. Rather than defending branded packages, PBT asks which psychological processes are driving a specific client's suffering — then targets those directly. It has become the framework tying together modern ACT research.
If the 2015 conference planted a seed, process-based therapy is the tree that grew from it. Rather than asking "Does treatment package X work for diagnosis Y?", PBT asks: "Which psychological processes are driving this individual client's suffering, and which interventions target those processes?"
Steven Hayes and Stefan Hofmann formalized this framework in a 2018 paper in World Psychiatry, arguing that the field needed to move from diagnosis-based protocols to a network model of interacting processes [Hayes & Hofmann, 2018]. The six core ACT processes — cognitive defusion, acceptance, present-moment awareness, self-as-context, values, and committed action — became foundational to this new framework, joined by processes from other traditions such as self-compassion, emotion regulation, and attentional flexibility.
Why does process-based therapy matter for everyday practice?
- Personalization: Clinicians can tailor interventions to the specific processes maintaining a client's difficulties rather than following a one-size-fits-all manual.
- Transdiagnostic reach: The same process work (e.g., defusion from self-critical thoughts) applies across depression, anxiety, and eating disorders.
- Measurement: Brief validated measures like the AAQ-II, CompACT, and MPFI allow clinicians to track process change session by session.
- Integration: PBT provides a common language that lets ACT, CBT, DBT, and compassion-focused approaches speak to one another.
The Mayo Clinic and Cleveland Clinic have both quietly incorporated process-based framings into their integrated behavioral health protocols, reflecting the practical utility of this shift [Mayo Clinic, 2024].
Psychological Flexibility: The Central Construct Refined
Psychological flexibility — the capacity to stay present, open, and engaged with what matters even in the face of difficult internal experiences — was already the core outcome of ACT in 2015. A decade of research has refined our understanding of it in significant ways, positioning it as one of the strongest known predictors of mental wellbeing.
How well does flexibility predict wellbeing?
Longitudinal studies now consistently show that psychological flexibility predicts mental health outcomes above and beyond diagnostic symptoms. A large-scale study of over 8,000 adults published in 2020 found that psychological inflexibility was more strongly associated with anxiety, depression, and general distress than any single demographic or diagnostic variable [Levin et al., 2020]. Mental Health America has emphasized that flexibility-based skills training may be one of the highest-yield public health interventions available [MHA, 2023].
Better Measurement Tools
The original Acceptance and Action Questionnaire (AAQ-II) was widely used by 2015 but was criticized for conflating flexibility with general distress. The past decade produced a wave of improved measures — including the Multidimensional Psychological Flexibility Inventory (MPFI) and the CompACT — that measure flexibility and inflexibility as distinct constructs across all six core processes.
Flexibility Beyond the Clinic
Research has now demonstrated that psychological flexibility interventions improve outcomes in education, workplace performance, athletic performance, parenting, and chronic disease self-management. The World Health Organization's mental health action plan explicitly emphasizes building population-level psychological skills as a preventive public health strategy [WHO, 2022].
Digital ACT: A Pandemic-Accelerated Revolution
Digital ACT was a fringe curiosity in 2015 and a mainstream delivery format by 2025. Meta-analyses show internet-delivered ACT (iACT) produces effects comparable to face-to-face treatment for depression, anxiety, chronic pain, and tinnitus, though attrition remains a challenge without human coaching.
The COVID-19 pandemic dramatically accelerated adoption, but the empirical groundwork was already being laid at that pivotal conference. Multiple meta-analyses now show that internet-delivered ACT (iACT) produces effects comparable to face-to-face delivery for depression, anxiety, chronic pain, and tinnitus [Thompson et al., 2021]. Apps like ACT Companion, Woebot (which integrates ACT-consistent elements), and various self-guided programs have made core skills accessible to populations who would never enter a therapist's office. The NIH has funded multiple large-scale trials of digital ACT for underserved populations, including rural veterans and non-English-speaking communities [NIH, 2023].
Strengths and Limits of Digital ACT
- Strength: Scalable, low-cost, available 24/7, reduces stigma.
- Strength: Effective for mild-to-moderate presentations of depression, anxiety, and stress.
- Limit: Attrition is high — often 30–50% — without human guidance or coaching.
- Limit: Less evidence for severe presentations, active suicidality, or complex trauma.
Cultural Adaptation and Global Expansion
One of the most important post-2015 developments has been ACT's serious engagement with cultural adaptation. Early ACT research was heavily concentrated in Western, English-speaking populations. The past decade has produced culturally adapted ACT protocols and trials in Iran, Japan, China, India, Turkey, Uganda, and across Latin America.
The World Health Organization's Self-Help Plus program — an ACT-based group intervention designed for refugee and conflict-affected populations — has been trialed in Uganda, Turkey, and multiple European countries with promising results for reducing psychological distress [WHO, 2021]. This represents a striking evolution: ACT is no longer only a treatment delivered in therapy offices; it is a public health tool being deployed at scale for populations facing collective trauma.
Ongoing Challenges
Despite progress, the field acknowledges that most ACT research still overrepresents white, educated, middle-class participants. Calls for greater diversity in trials, culturally responsive training, and community-based participatory research have grown louder in the past five years. The American Psychological Association's 2024 guidelines on multicultural competence emphasize that evidence-based practice requires attention to cultural context, not just symptom checklists [APA, 2024].
The Six Core Processes: What's Changed in Application
The six core ACT processes — defusion, acceptance, present-moment awareness, self-as-context, values, and committed action — remain the backbone of practice. But a decade of research has sharpened how each is understood, measured, and delivered in session.
Cognitive Defusion
Once framed primarily through catchy exercises ("Milk, milk, milk"), defusion is now understood as a broader capacity to notice thoughts as mental events rather than literal truths. Research has clarified that defusion works by changing the function of thoughts, not their frequency or content — an important refinement that helps clients let go of unrealistic expectations about "stopping" intrusive thoughts.
Acceptance
The past decade has sharpened the distinction between acceptance, resignation, and tolerance. Contemporary ACT emphasizes that acceptance is an active willingness to experience internal events in service of values — not passive endurance or giving up.
Present-Moment Awareness
Mindfulness research from outside ACT has enriched this process significantly. The Harvard Medical School review of mindfulness-based interventions notes overlapping mechanisms with ACT's present-moment work, particularly reductions in default mode network activity associated with rumination [Harvard Health Publishing, 2023].
Self-as-Context
Perhaps the most theoretically debated process, self-as-context has been operationalized more precisely in recent years through hierarchical relational framing research and refined therapist protocols. It remains the most difficult process for both therapists and clients to work with, but its role in resilience and identity flexibility is increasingly clear.
Values
Values work has moved beyond simple worksheets to more sophisticated processes for uncovering vital directions, distinguishing values from goals, and using values as ongoing motivational resources. Values clarity is now understood as a strong predictor of therapy engagement and outcome.
Committed Action
The behavioral heart of ACT has been enriched by integration with habit science, behavioral activation, and implementation intention research. This has made ACT more effective at translating insight into sustained behavior change — historically a challenge for insight-oriented therapies.
Practical Takeaways for Clinicians and Learners in 2025
Whether you are training in ACT, integrating it into an eclectic practice, or curious as a client, the decade of research since 2015 offers several practical guides:
- Think processes, not protocols. Ask which psychological processes are keeping this person stuck, then choose interventions accordingly.
- Measure what you treat. Use brief validated flexibility measures (AAQ-II, CompACT, MPFI) at intake and throughout treatment.
- Match method to preference. Digital ACT, group formats, brief interventions, and traditional weekly therapy all work — client preference matters for engagement.
- Prioritize values early. Values clarity predicts engagement; front-loading values work in the first few sessions improves retention.
- Integrate rather than compete. ACT plays well with CBT, DBT, self-compassion, and trauma-focused work. Rigid brand loyalty is unhelpful.
- Attend to culture. Metaphors, values exercises, and language must be adapted for cultural and identity contexts.
- Track your own flexibility. Therapist psychological flexibility predicts client outcomes; ongoing personal practice matters.
What the Next Decade May Bring
Several trends visible in current research suggest where acceptance and commitment therapy research is heading:
- Precision matching: Machine learning models predicting which clients will benefit from which processes.
- Neuroscience integration: More sophisticated understanding of the brain networks underlying flexibility, defusion, and values-based action.
- Prevention at scale: Population-level ACT-based programs in schools, workplaces, and healthcare systems.
- Deeper trauma integration: Better protocols for combining ACT with somatic and trauma-focused approaches for complex trauma.
- Continued cultural expansion: More trials led by researchers from the Global South, with genuinely adapted rather than translated protocols.
Final Thoughts
The 2015 ACT conference did not by itself change the field — no single event ever does. But it captured a moment when the ACT community collectively decided to grow up: to submit its ideas to more rigorous testing, to abandon defensiveness in favor of process science, and to open itself to the possibility that being right about the model mattered less than helping people live meaningful lives. A decade later, the evidence base is stronger, the practice is more nuanced, and psychological flexibility has earned its place as one of the most robust predictors of human wellbeing yet identified.
For clinicians, the message is one of quiet confidence. For clients, it is one of hope: this is a therapy grounded not in slogans but in growing scientific rigor. And for the field of mental health more broadly, the ACT decade is a reminder that our best therapies are not fixed dogmas but living, evolving practices — always in service of the same core question ACT has asked from the beginning: given that life includes suffering, what kind of life do you want to build?
Frequently Asked Questions
Is acceptance and commitment therapy evidence-based?
Yes. ACT is recognized by the American Psychological Association's Division 12 as having strong or modest research support across depression, chronic pain, OCD, mixed anxiety, and psychosis. More than 900 randomized controlled trials support its use, and it is endorsed by SAMHSA and the U.S. Department of Veterans Affairs as an evidence-based practice.
What is the difference between ACT and CBT?
CBT typically targets the content of thoughts, working to modify or challenge distorted cognitions. ACT targets the relationship a person has with their thoughts, using defusion and acceptance so that difficult inner experiences no longer dictate behavior. Both are effective, and modern process-based approaches often blend them, matching techniques to the client's specific needs.
How long does ACT therapy usually take?
ACT is often delivered in 8 to 16 sessions, though brief interventions of 1 to 4 sessions have shown efficacy for specific problems like inpatient psychosis and smoking cessation. Longer-term ACT is common for complex presentations. Digital and group formats can extend or shorten this range depending on client goals.
Can ACT be done online or through an app?
Yes. Internet-delivered ACT (iACT) and app-based programs have accumulated strong meta-analytic support for depression, anxiety, chronic pain, and tinnitus. Digital ACT is most effective for mild-to-moderate presentations, particularly when supported by human coaching, and less appropriate for severe conditions or active safety concerns.
What is process-based therapy?
Process-based therapy (PBT) is a framework that focuses on identifying and targeting the specific psychological processes driving an individual client's suffering, rather than applying named treatment packages to diagnostic categories. It emerged from the 2015 ACT era and is now shaping the broader field of evidence-based psychotherapy.
Does ACT work for chronic pain?
Yes. ACT for chronic pain is classified by the APA as having strong research support. Rather than aiming to eliminate pain, ACT helps clients reduce pain interference, disability, and depression by increasing values-consistent action. A 2023 systematic review in Pain confirmed clinically meaningful improvements across multiple pain populations.
What is psychological flexibility?
Psychological flexibility is the capacity to stay present, open, and engaged with what matters even when difficult thoughts, feelings, or sensations arise. It is the central mechanism of change in ACT and one of the strongest known predictors of mental health and wellbeing across research populations.
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