"In this paper we study experimentally four remedies to overcome inefficiencies that arise from the incompleteness of contracts. These remedies are reciprocity, repeated game effects, social embeddedness, and incentive contracts. In our baseline treatment we find that reciprocity is a powerful contract enforcement device. A second experiment establishes that repeated game effects interact with reciprocity in a complementary way, i.e., efficiency is increased compared to our baseline. Adding social approval incentives does not contribute significantly to efficiency. Finally, we show that explicit incentive contracts may have perverse effects in the sense that they ""crowd out"" reciprocity and therefore reduce efficiency compared to the baseline. In our concluding section we discuss the relation of our findings to the recent literature on ""intrinsic motivation""."