Abstract

A very popular explanation for the emergence of cooperation, a problematic phenomenon for a rational approach, is the existence of an underlying network of contacts constraining who one can interact with. A wealth of theoretical studies have concluded during the last twenty years that this ``network reciprocity’’ is indeed possible under a variety of circumstances. In particular, simulations indicate that heterogeneous networks should be particularly efficient in fostering cooperation in social dilemmas. In order to put these theoretical results through the test of experiments beyond the few available laboratory works on very small networks, we studied two large networks, with 1229 human subjects, placed either on a square lattice or on a scale-free network. Despite their very different structure, the level of cooperation reached was indistinguishable in both networks and comparable to that found in smaller ones or in unstructured populations. Our results exclude network reciprocity as a valid hypothesis to explain the emergence of cooperation among humans involved in a Prisoner’s Dilemma, mostly because humans seem to disregard payoff differences with their neighbours, adopting a more reciprocal attitude.