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Sci Rep. 2015 Jul 15;5:12145. doi: 10.1038/srep12145.

Stability of cooperation under image scoring in group interactions.

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Department of Social Sciences, ETH Zürich, 8092 Zürich, Switzerland.
1] Department of Physics, Faculty of Sciences, King Abdulaziz University, Jeddah, Saudi Arabia [2] Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška cesta 160, SI-2000 Maribor, Slovenia.
Institute of Technical Physics and Materials Science, Centre for Energy Research, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary.
1] Computational Social Science, ETH Zürich, 8092 Zürich, Switzerland [2] Risk Center, ETH Zürich, 8092 Zürich, Switzerland.


Image scoring sustains cooperation in the repeated two-player prisoner's dilemma through indirect reciprocity, even though defection is the uniquely dominant selfish behaviour in the one-shot game. Many real-world dilemma situations, however, firstly, take place in groups and, secondly, lack the necessary transparency to inform subjects reliably of others' individual past actions. Instead, there is revelation of information regarding groups, which allows for 'group scoring' but not for image scoring. Here, we study how sensitive the positive results related to image scoring are to information based on group scoring. We combine analytic results and computer simulations to specify the conditions for the emergence of cooperation. We show that under pure group scoring, that is, under the complete absence of image-scoring information, cooperation is unsustainable. Away from this extreme case, however, the necessary degree of image scoring relative to group scoring depends on the population size and is generally very small. We thus conclude that the positive results based on image scoring apply to a much broader range of informational settings that are relevant in the real world than previously assumed.

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