Cooperation, but No Reciprocity: Individual Strategies in the Repeated Prisoner’s Dilemma
分析了四个近期实验中个体在重复囚徒困境中的策略，发现“半冷酷”策略（合作后合作、背叛后背叛、否则随机）能很好解释行为，并解释了合作出现的折扣因子阈值。
In the repeated prisoner's dilemma, predictions are notoriously difficult. Recently, however, Blonski, Ockenfels, and Spagnolo (2011)—henceforth, BOS—showed that experimental subjects predictably cooperate when the discount factor exceeds a particular threshold. I analyze individual strategies in four recent experiments to examine whether strategies are predictable, too. Behavior is well summarized by “Semi-Grim” strategies: cooperate after mutual cooperation, defect after mutual defection, randomize otherwise. This holds both in aggregate and individually, and it explains the BOS-threshold: Semi-Grim equilibria appear as the discount factor crosses this threshold, and then, subjects start cooperating in round 1 and switch to Semi-Grim in continuation play.