Hume made the case for this process absolutely clear: When each individual perceives the same sense of interest in all his fellows, he immediately performs his part of any contract, as being assur'd that they will not be wanting in theirs. All of them, by concert enter into a scheme of actions, calculated for common benefit, and agree to be true to their words; nor is there anything requisite to form this concert or connection, but that every one have a sense of interest in the faithful fulfilling of engagements, and express that sense to other members of the society. This immediately causes that interest to operate upon them and interest is the first obligation to the performance of promises.

XREF: Connects to game theory and the prisoner's dilemma — mutual expectation of cooperation sustains social contracts. Also echoes Hobbes's account of covenants and Ostrom's work on commons governance.

Diego Gambetta, Trust - Making and Breaking…, loc. 2795