The chapter discusses whether, and if so how, empirical insights can be integrated into law, thereby potentially challenging its behavioural assumptions. Law as a normative social practice has distinct features which are not easily reconciled with the view of law as social engineering. These distinct features are closely related to law’s assumptions about human agency. The idea of law as normative guidance to reasonable agents is in tension with naturalistic models of human behaviour and regulatory ideas, such as nudging and libertarian paternalism, supposedly following from these findings. On the one hand, law seems to be committed to an idea of human agency. On the other hand, it is a regulatory technique in the service of policy goals. The tension between these two understandings of the general character of law has repercussions for how empirical insights about human behaviour can be integrated into law. Yet the tension can be reduced by acknowledging non-legal modes of governance or revising law’s counterfactual assumptions.