  Evolutionary game theory predicts that cooperation in social dilemma games is
promoted when agents are connected as a network. However, when networks are
fixed over time, humans do not necessarily show enhanced mutual cooperation.
Here we show that reinforcement learning (specifically, the so-called
Bush-Mosteller model) approximately explains the experimentally observed
network reciprocity and the lack thereof in a parameter region spanned by the
benefit-to-cost ratio and the node's degree. Thus, we significantly extend
previously obtained numerical results.
