 The concept of a Nash equilibrium (see the entry on  game theory)  has been the most used solution concept in game theory since its introduction by John Nash (1950). A selection of strategies by a group of agents is said to be in a Nash equilibrium if each agent’s strategy is a best-response to the strategies chosen by the other players. By best-response, we mean that no individual can improve her payoff by switching strategies unless at least one other individual switches strategies as well. This need not mean that the payoffs to each individual are optimal in a Nash equilibrium: indeed, one of the disturbing facts of the prisoner’s dilemma is that the only Nash equilbrium of the game—when both agents defect—is  suboptimal.[6]