This article develops an economic theory of expressive law. By expressing social values, law can tip a system of social norms into a new equilibrium. This process can create or destroy a social norm without changing individual values. In addition, law can change the individual values of rational people. Internalizing a social norm is a moral commitment that attaches a psychological penalty to a forbidden act. A rational person internalizes a norm when commitment conveys an advantage relative to the original preferences and the changed preferences. I call such a commitment a Pareto self-improvement. By creating opportunities for Pareto self-improvements, law induces rational people to change their preferences. Inducing change in this way respects individual preferences, rather favoring a particular moral theory.