Abstract The key context within which preschool children learn to justify beliefs with reasons is collaborative problem-solving and decision-making with peers, including in the moral domain, in which they engage with another coequal mind in a cooperative spirit. Evidence for this proposal comes from recent studies in which children demonstrated sensitivity to the common ground assumptions they shared with their peer partners in decision-making, as well as an ability to provide reasons relevant to their shared understanding. Training studies suggest that discourse with others about reasons for beliefs provides children with the appropriate learning experiences. Internalizing this communicative process may be crucial for individual deliberative reasoning.