How can democracies satisfy citizens' demands for legitimate decision making? This article reports findings from a randomised field experiment designed to mimic decision making in large‐scale democracies. Natural collectives of individuals with a shared history and future (high school classes) were studied. They were asked to make a decision about how to spend a sum of money under arrangements imposed by the researchers and distributed randomly across classes. Within this setting, empirical support for three ideas about legitimacy enhancing decision‐making arrangements is tested: participatory constitution‐making; personal involvement in the decision‐making process; and fairness in the implementation of arrangements. Throughout the analyses it was found that personal involvement is the main factor generating legitimacy beliefs.