People act upon their desires, but often, also act in adherenceto implicit social norms. How do people infer these unstatedsocial norms from others’ behavior, especially in novel so-cial contexts? We propose that laypeople have intuitive the-ories of social norms as behavioral constraints shared acrossdifferent agents in the same social context. We formalize in-ference of norms using a Bayesian Theory of Mind approach,and show that this computational approach provides excellentpredictions of how people infer norms in two scenarios. Ourresults suggest that people separate the influence of norms andindividual desires on others’ actions, and have implications formodelling generalizations of hidden causes of behavior.