Cooperation in temporary partnerships
研究了在对手并非每期都变、而是偶尔变化的临时合作关系中，如何通过直接惩罚和社区惩罚来维持合作，并找到了比传统“冷酷”策略更有效的策略。
The literature on cooperation in infinitely repeated Prisoner's Dilemmas covers the extreme opposites of the matching spectrum: partners , a player's opponent never changes, and strangers , a player's opponent randomly changes in every period. Here, we extend the analysis to settings where the opponent changes, but not in every period. In these temporary partnerships, players can deter some deviations by directly sanctioning their partner. Hence, relaxing the extreme assumption of one-period matchings can support some cooperation also off equilibrium because a class of strategies emerges that are less extreme than the typical “grim” strategy. We establish conditions supporting full cooperation as a subgame perfect equilibrium under a social norm that complements direct sanctions with a cyclical community sanction. Though this strategy less effectively incentivizes cooperation, it more effectively incentivizes punishment after a deviation, hence, can be preferable to the grim strategy under certain conditions.