Laws of promise keeping can have substantial force for exchange promises but little or none for coordination promises. They commonly have little force for gratuitous promises—unless there is subsequent reliance that gives the promisee a loss relative to the status quo ante if the promise is not fulfilled. Even for exchange promises, the law usually enters only if the stakes are relatively high and there has been some reliance that would prove costly if the promise were not fulfilled.

DEFINE: Distinguishes exchange, coordination, and gratuitous promises by how much legal force each receives.

Russell Hardin, Trust and Trustworthiness, loc. 327