We propose a simple theory of social norms that models the distinct influence on behavior of personal values, normative expectations and empirical expectations. The first and second moments of the distribution of normative expectations affect the strength of social norms' pull on behavior. We test the empirical predictions of the model through an experiment based on a variant of the dictator game. Consistent with the theory, we find that normative expectations influence generosity and that higher dispersion of such expectations leads to more variation in giving behavior.