Does Group Size Matter in Therapy? New Research Reveals Surprising Gaps in What We Know

When mental health professionals set up group therapy sessions, one of the first decisions they face is how many people to include. Should it be an intimate circle of four or five participants, or a larger gathering of ten or twelve? Despite decades of expert opinions on this topic, a new systematic review reveals that science has surprisingly little concrete evidence to guide these crucial decisions.

Researchers analyzed 17 studies involving over 21,000 participants to examine whether group size actually affects how well therapy works. The results paint a picture of incomplete knowledge in an area where definitive answers are badly needed.

Mixed Results Across Studies

The review found that only seven of the 17 studies showed any significant relationship between group size and therapy outcomes. This inconsistency suggests that the impact of group size may be more complex and nuanced than previously assumed.

Among the studies that did find meaningful differences, most pointed toward potential benefits of smaller groups. However, one study of cognitive behavioural therapy for chronic pain favoured larger groups. At the same time, another suggested that groups with five to nine members performed better than either smaller or larger alternatives.

Different Types of Therapy May Respond Differently

One intriguing pattern emerged when researchers looked at different therapeutic approaches. Groups focused on interpersonal dynamics and emotional processing appeared more sensitive to size changes than structured cognitive behavioural therapy sessions.

This makes intuitive sense when considering how these therapies operate. Process oriented approaches rely heavily on group interaction, personal sharing, and relationship building among participants. In contrast, cognitive behavioural therapy often follows structured protocols with specific skills training components.

The Quality Problem

A concerning finding was the overall quality of available research. Most studies examining group size effects were rated as low quality, with significant limitations in their design and methodology. Only four studies met criteria for high quality research standards.

This research gap means that mental health professionals must rely primarily on clinical experience, traditional practices, and expert recommendations when deciding on group composition. While these sources of guidance have value, they lack the rigorous testing that evidence based practice demands.

What the Trends Suggest

Despite the limited evidence, some tentative patterns did emerge from the analysis. Groups with fewer than nine members showed slight tendencies toward better outcomes compared to larger groups. This aligns roughly with the upper limits that many therapy experts have traditionally recommended over the years.

The research also suggested that clinical settings might see more pronounced group size effects than university or community programs. This could reflect differences in participant needs and functioning levels across these environments.

The Path Forward

The researchers emphasize that much more investigation is needed before making firm recommendations about optimal group sizes. They suggest several approaches for future study, including direct comparisons between different sized groups within the same treatment programs and examination of how various factors might influence size effects.

Questions remain about whether certain personality types or attachment styles make individuals more or less sensitive to group size. The role of multiple facilitators in accommodating larger groups also deserves exploration, as does the potential for different optimal sizes across various mental health conditions.

Implications for Practice and Policy

These findings have practical implications beyond individual therapy decisions. Insurance companies and healthcare systems need evidence based guidelines to set appropriate reimbursement rates and service standards. Without solid research, these important policy decisions rest on uncertain foundations.

The review also highlights opportunities for relatively simple research that could provide valuable answers. Treatment centers could systematically vary group sizes and track outcomes, or analyze data from existing programs that naturally include groups of different sizes.

A Call for Better Evidence

While the current research provides some useful insights, it primarily reveals how much we still don't know about this fundamental aspect of group therapy. The authors argue that decisions affecting thousands of therapy participants should be based on rigorous scientific evidence rather than tradition and expert opinion alone.

This doesn't diminish the value of clinical experience and established practices, but it does suggest that the mental health field would benefit significantly from more systematic investigation of group size effects. As therapy continues to evolve and expand, having solid empirical foundations for basic structural decisions becomes increasingly important.

The incomplete picture presented by current research serves as both a limitation and an opportunity. While therapists and program administrators must continue making group size decisions with imperfect information, the identified research gaps provide clear directions for studies that could meaningfully improve therapeutic outcomes for countless individuals seeking help through group interventions.

Twomey, C., & Dowling, C. (2025). Associations of group size with cohesion and clinical outcomes in group psychotherapy: a systematic review. International Journal of Group Psychotherapy, 75(2), 345-364.