Promises, Expectations, and Social Cooperation∗

Promising serves as an important commitment mechanism by operating on a potential cheater’s internal value system. We present experimental evidence on what motivates people to keep their promises. First, they feel that they are duty-bound to keep their promises regardless of whether promisees expect them to (promising per se effect). Second, they care about not disappointing promisees’expectations, regardless of whether those expectations were induced by the promise (expectations per se effect). Third, they are even more motivated to avoid disappointing promisees’expectations when those expectations were induced by a promise (interaction effect). Clear evidence of some of these effects has eluded the prior literature due to limitations inherent to the experimental methods employed. We sidestep those diffi culties by using a novel between-subject vignette design. Our results also shed light on how promising may contribute to the self-reinforcing creation of trust.