Unbeatable Imitation

We show that for many classes of symmetric two-player games, the simple decision rule "imitate-the-best" can hardly be beaten by any other decision rule. We provide necessary and sufficient conditions for imitation to be unbeatable in the sense that, even against a very clever opponent, imitation is subject to a money pump if and only if the relative payoff function of the game is of the rock-scissors-paper variety. For many interesting classes of games including examples like 2x2 games, Cournot duopoly, price competition, public goods games, common pool resource games, and minimum effort coordination games, we obtain an even stronger notion of the unbeatability of imitation.