Heterogeneous Groups Overcome the Diffusion of Responsibility Problem in the Volunteer’s Dilemma

The evolution of cooperation in humans has been studied by means of linear, symmetric collective goods games played by homogenous actors. However, many cooperation problems human groups face are non-linear and asymmetric. These cooperation problems can be described with step-level collective goods games played by heterogeneous actors. The volunteer’s dilemma (VOD) is a binary choice, n-person game in which a single actor’s cooperation is necessary and sufficient to provide the collective good for the entire group. An interesting property of the symmetric VOD is that actors’ probabilities to cooperate decrease with group size. This property is also known as “diffusion of responsibility” or “bystander” effect. In the asymmetric VOD, which differs from the symmetric version in one actor having lower costs of cooperation, groups coordinate on the “strongest” actor to provide the collective good alone, irrespective of group size. We study the diffusion of responsibility in social norm enforcement (i.e. second-order cooperation) by means of the VOD. In a computerized laboratory experiment we show how larger groups are less effective in sanctioning a norm breaker if the sanctioning situation is symmetric. In the asymmetric condition it is mostly the actor with the lowest costs who sanctions. As a consequence, the diffusion of responsibility effect is reduced in the asymmetric condition leading to more first-order cooperation than in the symmetric condition. Our results show that heterogeneous groups can be more successful in achieving cooperation than groups of all equals because they naturally evade diffusion of responsibility. More generally, our results show how the studying of non-linear, asymmetric cooperation problems can unveil hitherto unknown mechanisms by which human groups achieve cooperation.