Chapter 65 Payoff Uncertainty and Cooperation in Finitely repeated Prisoners Dilemma Games Several of the results reported here broadly support theories of sequential equilibria. In particular at least four pieces of data indicate that game theorys structurally oriented models predict some of the reasons for individuals play in the prisoners dilemma game: (1) more cooperation resulted when a cooperative equilibrium existed; (2) players cooperated more overall and more in the first round in SW than in SS; (3) Column was more cooperative than Row on the first trial; and (4) the SW Column Uncertain condition produced the most end game cooperation. Unfortunately these effects only explain part of the picture. In essence even after considering the effects of uncertainty the original issue still remains: considerable cooperation resulted when no cooperative equilibria exist.