The ability to select and reject partners creates a powerful means of supporting cooperation when a common set of actors faces repeated possibilities for playing the prisoner’s dilemmas with each other, a common situation that we refer to as a voluntary dilemma. The cooperative quit-for-tat (QFT) strategy that maintains all relationships with mutually cooperative partners but quits any relationship after a defection can maintain cooperation in voluntary dilemmas by joining together and excluding nasty and exploitative strategies. We develop the implications of the QFT model for the dynamics and structure of mutual cooperation, and test these implications in an experimental voluntary dilemma. The results confirm that the simple QFT model accounts for observed dynamics and structure of cooperative relationships, and that high-scoring subjects follow strategies that resemble QFT. We discuss the relative importance of niceness, forgiveness, and optimistic search in accounting for the success of QFT strategies, and note that the observed clustering of cooperators in this experimental setting is an artifact rather than a necessary support for cooperation.