title: Static network structure can stabilize human cooperation.
abstract: The evolution of cooperation in network-structured populations has been a major focus of theoretical work in recent years. When players are embedded in fixed networks, cooperators are more likely to interact with, and benefit from, other cooperators. In theory, this clustering can foster cooperation on fixed networks under certain circumstances. Laboratory experiments with humans, however, have thus far found no evidence that fixed network structure actually promotes cooperation. Here, we provide such evidence and help to explain why others failed to find it. First, we show that static networks can lead to a stable high level of cooperation, outperforming well-mixed populations. We then systematically vary the benefit that cooperating provides to one's neighbors relative to the cost required to cooperate (b/c), as well as the average number of neighbors in the network (k). When b/c > k, we observe high and stable levels of cooperation. Conversely, when b/c ≤ k or players are randomly shuffled, cooperation decays. Our results are consistent with a quantitative evolutionary game theoretic prediction for when cooperation should succeed on networks and, for the first time to our knowledge, provide an experimental demonstration of the power of static network structure for stabilizing human cooperation.
authors.affiliation: Departments of Psychology, Economics, School of Management, and Yale Institute for Network Science, Yale University, New Haven, CT 06511; david.rand@yale.edu.
authors.forename: David G
authors.initials: DG
authors.lastname: Rand
authors.affiliation: Program for Evolutionary Dynamics and Departments of Mathematics and Organismic Biology, Harvard University, Cambridge, MA 02138; and.
authors.forename: Martin A
authors.initials: MA
authors.lastname: Nowak
authors.affiliation: Medical Genetics Division and Political Science Department, University of California, San Diego, CA 92093.
authors.forename: James H
authors.initials: JH
authors.lastname: Fowler
authors.affiliation: Yale Institute for Network Science, Yale University, New Haven, CT 06511; Sociology, Medicine, and Ecology and Evolutionary Biology.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Christakis
mesh_terms: D000328:Adult; D005075:Biological Evolution; D003299:Cooperative Behavior; D005716:Game Theory; D005717:Games, Experimental; D006801:Humans; D007398:Interpersonal Relations; D008960:Models, Psychological; D011582:Psychological Theory; D055815:Young Adult
keywords: Prisoner’s Dilemma; assortment; economic games; evolutionary game theory; structured populations