Heterogeneous networks do not promote cooperation when humans play a Prisoner’s Dilemma

TLDR

Human cooperation with strangers remains poorly understood, and various hypotheses have been proposed to explain large‑scale cooperation in increasingly complex global interaction networks. The study aims to experimentally test whether population structure promotes cooperation in large human Prisoner’s Dilemma games. The authors conducted the largest human experiments to date, having 1,229 participants play a spatial Prisoner’s Dilemma on both a lattice and a scale‑free network. Cooperation levels were identical on both lattice and scale‑free networks, and participants responded reciprocally to observed cooperation rather than neighbors’ payoffs, indicating that population structure has little influence on human cooperation.

Abstract

It is not fully understood why we cooperate with strangers on a daily basis. In an increasingly global world, where interaction networks and relationships between individuals are becoming more complex, different hypotheses have been put forward to explain the foundations of human cooperation on a large scale and to account for the true motivations that are behind this phenomenon. In this context, population structure has been suggested to foster cooperation in social dilemmas, but theoretical studies of this mechanism have yielded contradictory results so far; additionally, the issue lacks a proper experimental test in large systems. We have performed the largest experiments to date with humans playing a spatial Prisoner’s Dilemma on a lattice and a scale-free network (1,229 subjects). We observed that the level of cooperation reached in both networks is the same, comparable with the level of cooperation of smaller networks or unstructured populations. We have also found that subjects respond to the cooperation that they observe in a reciprocal manner, being more likely to cooperate if, in the previous round, many of their neighbors and themselves did so, which implies that humans do not consider neighbors’ payoffs when making their decisions in this dilemma but only their actions. Our results, which are in agreement with recent theoretical predictions based on this behavioral rule, suggest that population structure has little relevance as a cooperation promoter or inhibitor among humans.