The tremendous costs of conflict have made humans resourceful not only at warfare but also at peacemaking. Although third parties have acted as peacemakers since the dawn of history, little is known about voluntary, informal third-party intervention in conflict. Here we introduce the Peacemaker Game, a novel experimental paradigm, to model and study the interdependence between disputants and third parties in conflict. In the game, two disputants choose whether to cooperate or compete and a third party chooses whether or not to intervene in the conflict. Intervention introduces side payments that transform the game disputants are playing; it also introduces risk for the third party by making it vulnerable to disputants' choices. Six experiments revealed three robust effects: (i) The mere possibility of third-party intervention significantly increases cooperation in interpersonal and intergroup conflicts; (ii) reducing the risk to third parties dramatically increases intervention rates, to every-one's benefit; and (iii) disputants' cooperation rates are consistently higher than third parties' intervention rates. These findings explain why, how, and when self-interested third parties facilitate peaceful conflict resolution.
