Ex-post opportunistic behavior, commonly present in bilateral trade relationships, is a key element of the transaction cost economics. Investment in outside options is a prime example of such opportunism and often leads to inefficiency, for example by exerting effort to search for alternative business partners even if it does not add trade value. We experimentally investigate a bilateral trade relationship in which standard theory assuming self-regarding preferences predicts that the seller will be better off by investing in the outside option to improve his bargaining position. The seller's investment, however, might negatively affect the buyer's other-regarding preferences if the investment is viewed as opportunistic. We find overall support for our hypotheses that arise from the link between other-regarding behavior and opportunism. Our findings suggest that when the transaction cost economics approach is applied to the design of a governance structure, other-regarding preferences should be taken into account. -post opportunistic behavior leads to ex-post inefficiency. This is significantly different from ex-ante inefficiency, the focus of the property-right theory of the firm (Grossman and Hart 1986;Hart and Moore 1990;Hart 1995). In the property-rights theory, the surplus (appropriable-quasi rents) created by relation-specific investment is shared between two parties through efficient bargaining. The surplus-sharing leads to inefficiency in ex-ante investment when contracts are incomplete, and the theory studies the roles of asset ownership in mitigating this ex-ante inefficiency. See Whinston (2003) and Gibbons (2005) for clear discussions on the differences between the transaction cost economics and the property-right theory of the firm. See 65 5 The derivation of conjectures based on the logic of the Revealed Altruism theory (Cox, Friedman, and Sadiraj 2008) is presented in Appendix A.