Cooperation in viscous populations - Experimental evidence

We experimentally investigate the effect of imperfect separation of groups (population viscosity) on group selection and cooperation in a standard prisoner’s dilemma environment. Subjects can repeatedly choose between two groups, that differ in the defector gain in the associated prisoner’s dilemma. Choosing into the group with the smaller defector-gain can signal one’s willingness to cooperate. The degree of separation of the two groups is varied between treatments. We find that both, the share of agents that choose into the ”cooperative” group and the share of agents that cooperate, rise sharply with the degree of group separation. Furthermore we estimate the subjects’ intrinsic willingness to cooperate as a function of the expected probability of their match cooperating. Only if viscosity is high ”enough” these functions are strictly increasing.