In conditions of social uncertainty—i.e., when exchanges are subject to the hazards of opportunism—committed relationships promote both the shadow of the future and the shadow of the past necessary for cooperation. For this reason, some argue that exchanges can be selfgoverned without the need of legal enforcement and other formal controls. While this conclusion is correct when the value of a long-term relationship does not vary much over time, we provide new experimental evidence showing that it is invalid when individuals face high exchange value uncertainty—i.e., when there are constantly new opportunities to transact with more valuable partners outside committed circles. Under both social and exchange value uncertainty, a reduction in commitment can potentially increase exchange performance, at the cost of a potential reduction in cooperation. By creating safeguards in market exchanges, contract enforcement can make individuals more willing to move out of committed relationships and into relationships with higher exchange value.