title: Social experiments in the mesoscale: humans playing a spatial prisoner's dilemma.
abstract: BACKGROUND
  The evolutionary origin of cooperation among unrelated individuals remains a key unsolved issue across several disciplines. Prominent among the several mechanisms proposed to explain how cooperation can emerge is the existence of a population structure that determines the interactions among individuals. Many models have explored analytically and by simulation the effects of such a structure, particularly in the framework of the Prisoner's Dilemma, but the results of these models largely depend on details such as the type of spatial structure or the evolutionary dynamics. Therefore, experimental work suitably designed to address this question is needed to probe these issues.
  
  
  RESULTS
  We have designed an experiment to test the emergence of cooperation when humans play Prisoner's Dilemma on a network whose size is comparable to that of simulations. We find that the cooperation level declines to an asymptotic state with low but nonzero cooperation. Regarding players' behavior, we observe that the population is heterogeneous, consisting of a high percentage of defectors, a smaller one of cooperators, and a large group that shares features of the conditional cooperators of public goods games. We propose an agent-based model based on the coexistence of these different strategies that is in good agreement with all the experimental observations.
  
  
  CONCLUSIONS
  In our large experimental setup, cooperation was not promoted by the existence of a lattice beyond a residual level (around 20%) typical of public goods experiments. Our findings also indicate that both heterogeneity and a "moody" conditional cooperation strategy, in which the probability of cooperating also depends on the player's previous action, are required to understand the outcome of the experiment. These results could impact the way game theory on graphs is used to model human interactions in structured groups.
authors.affiliation: Grupo Interdisciplinar de Sistemas Complejos, Departamento de Matemáticas, Universidad Carlos III de Madrid, Leganés, Madrid, Spain.
authors.forename: Jelena
authors.initials: J
authors.lastname: Grujić
authors.forename: Constanza
authors.initials: C
authors.lastname: Fosco
authors.forename: Lourdes
authors.initials: L
authors.lastname: Araujo
authors.forename: José A
authors.initials: JA
authors.lastname: Cuesta
authors.forename: Angel
authors.initials: A
authors.lastname: Sánchez
mesh_terms: D003299:Cooperative Behavior; D005717:Games, Experimental; D006801:Humans; D007398:Interpersonal Relations; D008960:Models, Psychological; D012919:Social Behavior; D011795:Surveys and Questionnaires