We have recorded which strategies players use when they cooperate in both games (at least 2/3 cooperation rate). In the control treatment (red), players adopt strategies consistent with Generous Tit-for-Tat 9 , 10 . In the same-partner treatment (yellow), they only cooperate if the co-player previously cooperated in both games. In the different-partner treatment (green), individuals are still most cooperative after receiving cooperation in both games, but there is overall less cooperation (and very little linkage). C and D represent the decision to cooperate or to defect, respectively.
