Hart and Moore (2008) argue that varying degrees of flexibility in contracts induce differing reference points and aspiration levels for parties’ shares of a transaction’s total surplus. As a consequence, a trade-off between adaptational flexibility and the prevention of distributional conflicts emerges. In a recent paper, Fehr et al. (2009b) analyze a buyer-seller-relationship with incomplete contracts and ex ante uncertainty regarding the sellers’ cost level to test these effects. We re-run their experiment and introduce another treatment with exogenously determined contract types. Like FHZ we find reference point effects in both treatments. However, uncooperative shading behavior in our treatments differs substantially from that described in FHZ. Furthermore, it makes a significant difference whether contract types are determined by buyers or determined exogenously. We explain this by introducing two further effects, a reciprocity effect and a signaling effect.