Objective: Group cohesion, as a central explanatory concept in the study of groups, has provided a foundation for explaining a number of group processes that have occupied researchers' attention since the founding of the journal Group Dynamics: Theory, Research, and Practice 25 years ago. Method: This review summarizes that research, focusing on three specific topics: group performance, the effectiveness of group psychotherapy, and group-based identity processes. Results: Theory and research in these areas have yielded substantial insights into cohesion and related group processes, but definitional, methodological, and theoretical uncertainties remain to be resolved. Conclusions: Seeking to resolve some of these uncertainties, an integrative model of cohesion that defines cohesion as a group's degree of unity-and distinguishes between the causes (or antecedents) of that unity (such as attraction bonds or group pride) and indicators of cohesion-is described, as is a short inventory that can be used to assess members' perceptions of the unity of their groups.
Highlights and Implications• The concept of cohesion is central to understanding groups and group processes, and recent research has yielded substantial advances in understanding the nature, the sources, and the consequences of this key group-level process.• Specifically, recent research provides a clearer understanding of when cohesion facilitates team performance, cohesion's influence on the outcome of group psychotherapy, and the relationship between cohesion and members' social identities.• To resolve continuing issues of definition and assessment, an integrative conceptualization of cohesion, unitary cohesion theory, is described; it defines cohesion as the unity of a group, and so distinguishes between the causes of cohesion, indicators of cohesion, and cohesion itself.• Recommendations for future research include identifying and documenting the type of cohesion assessed in specific instruments and using methods that are appropriate when studying a group-level process.• A unitary cohesion index is described. This set of bipolar rating scales can be used to directly assess members' estimates of their group's cohesion by separating the defining conditions of cohesion from the potential causes of cohesion.