In game theory, an outcome or set of strategies, one for each player, is in Nash equilibrium if it yields an outcome such that no player can improve her position by unilaterally withdrawing from it. A game may permit several Nash equilibria. In the prisoners’ dilemma the strategy that each cheat on the other is, unfortunately, a Nash equilibrium, whereas the socially preferable outcome that each cooperates is not. See also Pareto principle.