This paper takes up David Gauthier's most recent defense of the rationality of cooperation in prisoner's dilemmas. In that defense, Gauthier argues for a Pareto-optimizing theory of rational choice. According to Gauthier, rational action should sometimes aim at Pareto-optimization, and cooperation in prisoner's dilemmas is rational because it is Pareto-optimizing. I argue that Pareto-optimization cannot justify cooperation in the prisoner's dilemma in a manner that is also consistent with Gauthier's other desiderata. Either: the rationality of cooperation must derive from what (...) is beneficial for the group rather than what is beneficial for the individual, leaving the individual qua individual without any reason to cooperate, or Gauthier cannot explain why defecting in prisoner's dilemmas is not also rational. (shrink)