  Prisoner’s Dilemma has no coordination equilibrium. Yet Sugden argues that the following “tit-for-tat” strategy might emerge as a convention when players repeatedly play Prisoner’s Dilemma over some indefinite period (e.g., two academics with a standing arrangement to exchange houses every summer): co-operate as long as your opponent cooperates; if your opponent defects, then defect for some prescribed number of rounds \(r\) as retaliation before cooperating again; if your opponent cooperates but you defect by mistake, then accept your opponent’s punishment in the next \(r\) rounds without retaliating. This equilibrium is not a convention in Lewis’s sense, since one always prefers that one’s opponent cooperate rather than defect.