Social norms promote cooperation in everyday life because many people are willing to negatively sanction norm breakers at a cost to themselves. However, a norm violation may persist if only one person is required to sanction the norm breaker and everyone expects someone else to do it. Here we employ the volunteer’s dilemma game (VOD) to model this diffusion of responsibility in social norm enforcement. The symmetric VOD is a binary choice game in which all actors have the same costs of and benefits from cooperation and only one actor’s cooperation is required to provide the collective good for the group. The asymmetric VOD differs from the symmetric VOD in one (strong) actor having lower costs of cooperation. In a laboratory experiment, we find that, in line with the diffusion of responsibility hypothesis, subjects’ propensities to sanction the norm breaker decrease with group size in the symmetric VOD. In the asymmetric VOD, groups tacitly coordinate on the strong subject to sanction the norm breaker alone. Although at first lower in larger groups, strong subjects’ sanctioning rates increase over time and reach equally high levels across different group sizes. Our results show that heterogeneous groups can be more effective in achieving norm compliance than groups of all equals because they naturally evade diffusion of responsibility in social norm enforcement.
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