Hart and John Moore (2008) introduce new behavioral assumptions that can explain long-term contracts and the employment relation. We examine experimentally their idea that contracts serve as reference points. The evidence confirms the prediction that there is a trade-off between rigidity and flexibility. Flexible contracts-which would dominate rigid contracts under standard assumptions-cause significant shading in ex post performance, while under rigid contracts much less shading occurs. The experiment appears to reveal a new behavioral force: ex ante competition legitimizes the terms of a contract, and aggrievement and shading occur mainly about outcomes within the contract.