Growing Cooperation

Experimental evidence shows that in a repeated dilemma setting cooperation is more likely to become the norm in small matching groups than in large ones. This result holds even if cooperation is an equilibrium outcome for all investigated group sizes. But what happens if small matching groups are merged to become large ones? Our paper is based on the idea that due to norm spillovers, a large group created by a merger of small groups is more likely to cooperate than a large group of similar size that is created directly. We tested this idea experimentally in the context of an infinitely repeated prisoner’s dilemma game. We compared the cooperation behavior of groups that result from mergers of smaller groups with the cooperation behavior of groups with constant group size. We found that cooperation levels were significantly higher in large groups that resulted from gradual growth than in large groups of the same size that were directly created. Looking at the individual behavior, we see that more subjects develop a norm of unconditional cooperation when the group size increases than when it is already large from the beginning. Hence, our results confirm the idea that cooperation is much more likely to be achieved when groups grow from small to large than when large groups are formed directly.