ACT Processes and Mediation: Challenges and Solutions Research

Acceptance and Commitment Therapy (ACT) is built around a clear idea: psychological suffering is often driven by rigid patterns of avoidance and fusion with difficult thoughts and feelings. ACT aims to increase psychological flexibility through six core processes, including acceptance, cognitive defusion, present-moment awareness, self-as-context, values, and committed action.

But here’s the question researchers continue to wrestle with: Do these processes actually explain how ACT works?

A paper titled Acceptance and Commitment Therapy (ACT) Processes and Mediation: Challenges and How to Address Them takes a deep look at this issue. The article includes Jason Luoma, PhD, from Portland Psychotherapy, as a co-author. The paper examines why testing ACT’s mechanisms of change is more complicated than it may seem — and how researchers can sharpen future studies.

Why Mediation Matters

In psychotherapy research, mediation refers to identifying the processes that explain why a treatment works. For ACT, the central hypothesis is that increases in psychological flexibility mediate improvements in outcomes such as reduced depression, anxiety, or substance use.

If ACT reduces symptoms because it increases acceptance or defusion, then changes in those processes should statistically account for changes in outcomes. Establishing this link is important. It strengthens the theory, guides training, and helps refine interventions.

However, mediation research in ACT has faced significant obstacles.

Three Reasons Mediation Is Hard to Test in ACT

The paper outlines several major challenges that complicate mediation testing in ACT research.

1. Measurement Problems

ACT processes are complex and overlapping. For example, acceptance and defusion are related but distinct constructs. Many commonly used measures, such as global psychological flexibility scales, combine multiple processes into a single score.

This creates a problem. If researchers only measure broad flexibility, they may miss which specific process is doing the work. At the same time, breaking processes into separate measures risks redundancy or conceptual confusion.

The authors emphasize the need for more precise, process-specific measurement tools that align closely with ACT theory.

2. Timing of Assessments

To demonstrate mediation, changes in the mediator must occur before changes in the outcome. Many studies measure processes and symptoms at the same time points, which makes it difficult to establish temporal order.

For example, if acceptance and depression both improve between pre- and post-treatment, it is unclear which changed first. Without frequent and well-timed measurement, causal conclusions remain uncertain.

The authors argue for more fine-grained designs, including repeated measurement during treatment, to better capture how change unfolds.

3. Statistical and Design Limitations

Mediation analysis requires adequate statistical power and thoughtful design. Small sample sizes, limited assessment points, and failure to control for confounding variables weaken conclusions.

Additionally, many ACT studies compare ACT to waitlist controls rather than to other active treatments. This makes it harder to determine whether ACT-specific processes are uniquely responsible for change, or whether improvements reflect general therapeutic factors.

Moving Toward Stronger Process Research

Rather than dismissing mediation research as too messy, the authors propose concrete solutions.

Use multiple measures of key processes.

Combining self-report, behavioral, and physiological measures may offer a more robust understanding of change.

Increase frequency of assessment.

Measuring processes and outcomes weekly, or even session by session, can help clarify whether shifts in flexibility precede symptom reduction.

Compare ACT to strong active treatments.

Testing whether ACT processes mediate change more strongly in ACT than in other therapies strengthens claims of specificity.

Refine theory alongside data.

If certain processes consistently fail to mediate outcomes, it may signal a need to revise or clarify theoretical assumptions.

Why This Matters for Clinicians

For therapists using ACT, this research does not undermine the model. Instead, it sharpens it.

ACT is often described as process-based rather than protocol-driven. Understanding which processes are most powerful in which contexts allows clinicians to tailor interventions more effectively. For example, increasing present-moment awareness may be more critical in some cases, while values clarification may drive change in others.

Process research also supports training. If therapists know which mechanisms are most predictive of improvement, they can focus more deeply on cultivating those skills.

Where Process Research Is Heading

Psychotherapy research has increasingly shifted toward identifying mechanisms of change rather than simply asking whether a treatment works. ACT has been at the forefront of this movement.

This paper reminds us that testing mechanisms is difficult, but not impossible. With stronger measurement, better designs, and careful statistical modeling, the field can move closer to understanding how psychological flexibility leads to meaningful change.

As process-based therapy continues to grow, refining mediation research becomes essential. The goal is not just to confirm that ACT works, but to identify precisely which processes drive change — so clinicians can apply them with greater intention and researchers can keep testing theory against real-world data.

You can read the full article here.

For clinicians: Read the full paper for the authors’ specific recommendations on measurement, design, and analysis. To deepen your own ACT practice, explore Portland Psychotherapy’s ACT training and consultation offerings.