10.1177/1046496403254585 ARTICLE SMALL GROUP RESEARCH / October 2003 Arrow, Crosson / SELF-ORGANIZED GROUP FORMATION MUSICAL CHAIRS Membership Dynamics in Self-Organized Group Formation HOLLY ARROW University of Oregon SCOTT CROSSON University of Oregon Participants played four rounds of a social card game in which they formed groups to make card hands and earn money. When isolates—players left out when groups formed—earned nothing, self-organized groups frequently included these extra people, even though this decreased earnings for group members. Although group composition remained fluid, with time most populations settled into a pattern of including everyone. When isolates received a small welfare payment, group formation was less ordered and predictable and exclusion remained common. Although exact repeats of the same group membership were relatively rare in both conditions, group stability was associated with more cooperative behavior by members. When membership was unstable, members were more likely to make excessive claims on group earnings, increasing their individual payoff at the expense of fellow group members. Keywords: group formation; membership changes; self-organization; small groups; social dilemma Small groups are often assembled by powerful outsiders in a top- down fashion. Managers assign workers to teams, officers assign soldiers to military units, and teachers assign students to discussion groups. Many groups, however, are not built. Instead, they assem- ble themselves in an organic fashion based on the shared attraction 523 AUTHORS’ NOTE: This research was supported in part by Grant Number SES-9729320 from the Decision, Risk & Management Science Program of the National Science Founda- tion to the first author and co-PI John Orbell. An earlier version of this paper was presented at the Sunbelt International Social Networks Conference, Vancouver, Canada, in April of SMALL GROUP RESEARCH, Vol. 34 No. 5, October 2003 523-556 DOI: 10.1177/1046496403254585 © 2003 Sage Publications