Young children mostly keep, and expect others to keep, their promises

•Study 1 investigated young children’s understanding of others’ promises.•When a promise to the child was broken, s/he protested and referred to promise norms.•In Studies 2 and 3 children promised to complete a task and were distracted.•They persisted longer on a task and mentioned their own obligation after promising.•Young children feel obligated to keep promises and expect others to keep theirs. Promises are speech acts that create an obligation to do the promised action. In three studies, we investigated whether 3- and 5-year-olds (N=278) understand the normative implications of promising in prosocial interactions. In Study 1, children helped a partner who promised to share stickers. When the partner failed to uphold the promise, 3- and 5-year-olds protested and referred to promise norms. In Study 2, when children in this same age range were asked to promise to continue a cleaning task—and they agreed—they persisted longer on the task and mentioned their obligation more frequently than without such a promise. They also persisted longer after a promise than after a cleaning reminder (Study 3). In prosocial interactions, thus, young children feel a normative obligation to keep their promises and expect others to keep their promises as well.