 Speaking generally, one might say that a PD is a game in which a “cooperative” outcome obtainable only when every player violates rational self-interest is unanimously preferred to the “selfish” outcome obtained when every player adheres to rational self-interest. We can characterize the selfish outcome either as the result of each player pursuing its dominant (strongly dominant) strategy, or as the unique weak (strong) nash equilibrium. In a two move game the two characterizations come to the same thing—a dominant move pair is a unique equilibrium and a unique equilibrium is a dominant move pair. As the payoff matrix below shows, however, the two notions diverge in a game with more than two moves.