The form and regulation of contracts is of increasing importance to agricultural economists as farmers and agribusinesses increasing rely on contracts rather than markets to acquire inputs and sell outputs. We focus on the differences between the joint and individual surplus achievable under complete versus incomplete or relational contracts, where the latter are contracts that are not verifiable by a third party and must rely upon threat of termination in order to entice mutually satisfactory performance. Using an experimental market similar to Brown, Falk, and Fehr [Brown, M., A. Falk, and E. Fehr. Relational Contracts and the Nature of Market Interactions, Econometrica, 72 (2004):747-780] we replicate the general results found by these authors, including the qualitative findings that complete contracts dominate incomplete contracts in terms of social surplus generated and that incomplete contracts significantly deviate from the minimal levels of social surplus predicted by equilibrium models featuring purely self-interested agents. We extend the Brown, Falk, and Fehr results in a fundamental way: we explicitly link individual outcomes in relational contracts (e.g, surplus, prices, quality) to the nature of subjects' social preferences, which were measured by a separate experimental protocol that was implemented prior to the experimental trading session. We find subjects with other-regarding preferences enter into relational contracts that generate levels of social surplus similar to the surplus generated under complete contracts. Furthermore, subjects with other-regarding preferences tend to locate others with similar preferences and enter into long-term trading relationships that generate these higher surplus levels. We discuss the ramifications of the results for current regulatory efforts aimed at agricultural contracts.