This paper reports a 3-phase experiment on a stylized labor market. In the first two phases agents face simple games, which we use to estimate subjects’ social and reciprocity concerns, together with their beliefs. In the last phase four principals, who face four teams of two agents, compete by offering agents a contract from a fixed menu. Then, each agent selects one of the available contracts (i.e. he “chooses to work” for a principal). Production is determined by the outcome of a simple effort game induced by the chosen contract. We find that (heterogeneous) social preferences are significant determinants of choices in all phases of the experiment. Since the available contracts display a trade-off between fairness and strategic u...