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Abstract:
The presence of costly cooperation between otherwise selfish actors is not trivial. A prominent mechanism that promotes
cooperation is spatial population structure. However, recent experiments with human subjects report substantially lower
level of cooperation then predicted by theoretical models. We analyze the data of such an experiment in which a total of
400 players play a Prisoner’s Dilemma on a 4|4 square lattice in two treatments, either interacting via a fixed square lattice
(15 independent groups) or with a population structure changing after each interaction (10 independent groups). We
analyze the statistics of individual decisions and infer in which way they can be matched with the typical models of
evolutionary game theorists. We find no difference in the strategy updating between the two treatments. However, the
strategy updates are distinct from the most popular models which lead to the promotion of cooperation as shown by
computer simulations of the strategy updating. This suggests that the promotion of cooperation by population structure is
not as straightforward in humans as often envisioned in theoretical models.
