On the Structural Robustness of Evolutionary Models of Cooperation

This paper studies the structural robustness of evolutionary models of cooperation, i.e. their sensitivity to small structural changes. To do this, we focus on the Prisoner’s Dilemma game and on the set of stochastic strategies that are conditioned on the last action of the player’s opponent. Strategies such as Tit-For-Tat (TFT) and Always-Defect (ALLD) are particular and classical cases within this framework; here we study their potential appearance and their evolutionary robustness, as well as the impact of small changes in the model parameters on their evolutionary dynamics. Our results show that the type of strategies that are likely to emerge and be sustained in evolutionary contexts is strongly dependent on assumptions that traditionally have been thought to be unimportant or secondary (number of players, mutation-rate, population structure...). We find that ALLD-like strategies tend to be the most successful in most environments, and we also discuss the conditions that favor the appearance of TFTlike strategies and cooperation.