This paper studies individual and social motives in tax evasion. We build a simple dynamic model that incorporates these motives and their interaction. The social motives underpin the role of norms and are the source of the dynamics that we study. Our empirical analysis exploits the adoption in 1990 of a poll tax to fund local government in the United Kingdom, which led to widespread evasion. The evidence is consistent with the model's main predictions on the dynamics of evasion.