Cooperation In Viscous Populations - Experimental Evidence

We experimentally investigate the effect of population viscosity (an increased probability to interact with others of one's type or group) on 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 viscosity is varied across treatments. We find that viscosity produces an endogenous sorting of cooperators and defectors and persistently high rates ofcooperation. Higher viscosity leads to a sharp increase in overall cooperation rates and in addition positively affects the subjects' intrinsic willingness to cooperate.