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We assess in the laboratory the impact of promises on group decision-making. The gift-exchange game provides the testing ground for our experiment. When played between groups, inter-group cooperation and reciprocity represent a condition for efficiency in overall decision making. We find that promises have a significant positive effect on aggregate profits. We interpret these findings as if promises act as a trigger of social conformity, according to which groups adopt socially more desirable behavior even without face-to-face communication or discussion.