 The idea mentioned in the introduction that the PD models a problem of cooperation among rational agents is sometimes criticized because, in a true PD, the cooperative outcome is not a nash equilibrium. Any “problem” of this nature, the critics contend, would be an unsolvable one. (See for example, Sugden or Binmore 2005, chapter 4.5.) By changing the payoff structure of the PD slightly, so that the reward payoff exceeds the temptation payoff, we obtain a game where mutual cooperation, as well as mutual defection, is a nash equilibrium. This game is known as the stag hunt. It might provide a better model for situations where cooperation is difficult, but still possible, and it may also be a better fit for other roles sometimes assigned to the PD. More specifically, a stag hunt is a two player, two move game with a payoff matrix like that for the PD given in  section 1  where the conditions PD1 are replaced by: