Same Game, New Tricks

The aim of this article is to distinguish between strategies in the Iterated Prisoner’s Dilemma on the basis of their relative performance in a given population set. We first define a natural order on such strategies that disregards isolated disturbances, by using the limit of time-average payoffs. This order allows us to consider one strategy as strictly better than another in some population of strategies. We then determine a strategy σ to be ‘‘robust,’’ if in any population consisting of copies of two types of strategies, σ itself and some other strategy τ, the strategy σ is never worse than τ. We present a large class of such robust strategies. Strikingly, robustness can accommodate an arbitrary level of generosity, conditional on the strength of subsequent retaliation; and it does not require symmetric retaliation. Taken together, these findings allow us to design strategies that significantly lessen the problem of noise, without forsaking performance. Finally, we show that no strategy exhibits robustness in all population sets of three or more strategy types.