Publication: Social experiments in the mesoscale: humans playing a spatial prisioner's dilemma
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2010-11
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Public Library of Science (PLoS)
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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.
Methods and Findings: 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.
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PLoS One, vol. 5, n. 11, november 2010. Pp. 1-9