Effects of the Number of Opponents on the Evolution of Cooperation in the Iterated Prisoner's Dilemma

Various settings of iterated prisoner's dilemma (IPD) games have been studied to examine the evolution of cooperation among players in the literature. In recent studies, players are often spatially placed in a network. Each player plays the IPD game against its neighbors, which are defined by connections between players in the network. As in the case of cellular IPD games in a two-dimensional lattice, the number of opponents is usually very small in network-based IPD games. In this paper, we examine the effect of the number of opponents on the evolution of cooperation through computational experiments using various networks. The main feature of our computational experiments is to randomly choose a pre-specified number of opponents for each player from its neighbors in one setting and from all players in another setting. In this manner, we examine a wide range of specifications of the number of opponents.