The law, as opposed to the mere practice, of promise keeping is an instance of the substitution of institutions for trust relationships. That law primarily covers some classes of promises, especially exchange promises involving significant values in exchange, while neglecting others, especially small-scale exchange promises for which recourse to the law would be disproportionately expensive and coordination promises, in which there are no values in exchange. … The law takes over those areas in which there is significant value at stake and in which trustworthiness would be inherently less reliable, because it would face endgame incentives either in a once-only interaction or in an ongoing relationship faced with unusually high stakes in a single interaction.

XREF: Connects to game theory concepts of one-shot versus repeated games — endgame incentives and defection risk. DEFINE: Clarifies the role of contract law as institutional substitution for informal trust, covering selective promise classes.

Russell Hardin, Trust and Trustworthiness, loc. 328