The aim of this paper is to examine the effect of neighborhood structures on the evolution of cooperative behavior in a spatial IPD (Iterated Prisoner's Dlemma) game where every player is located in a cell of a two-dimensional grid-world. In our spatial IPD game, a player in each cell plays against players in its neighboring cells. A game strategy of each player is represented by a binary string, which determines the next action based on a finite history of previous rounds of the IPD game. A new strategy for a player is generated by genetic operations from a pair of parent strategies, which are selected from its neighbors. We use two neighborhood structures: One is for the interaction among players (i.e., IPD game) and the other is for the genetic operations. Simulation results show that the evolution of cooperative behavior is facilitated by small neighborhood structures for the genetic operations as well as for the IPD game. We also examine a variant of our spatial IPD game where an opponent of a player is randomly selected from its neighbors at every round of the IPD game. This means that the IPD game is not iterated against the same opponent.
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