Recent theoretical work on on incomplete contracts suggests that contracts may not only define trading parties' rights and obligations but may also have important psychological effects. In particular, it has been hypothesized that competitively negotiated ex ante contracts may provide salient reference points which shape perceived entitlements in ex post trade. A series of papers demonstrates that the existence of such contractual reference points has a number of important implications for the theory of the firm. We have conducted a series of controlled laboratory experiments testing the empirical relevance of the underlying behavioral assumptions of this new strand of literature. Our evidence is highly supportive for the hypothesis that contracts serve as reference points. Specifically, we find that there is an important trade-off between contractual rigidity and flexibility. While the existence of this trade-off is in line with the theory of contractual reference points, it is in strong contrast to both standard economic theory and established behavioral models of social preferences. Further experimental conditions also reveal that the central behavioral mechanism underlying the concept of contractual reference points is robust to the presence of informal agreements and ex post renegotiation.