The Coevolution of Behavior and Normative Expectations: Customary Law in the Lab

Customary law has been criticized from very different angles. Rational choice theorists claim that what looks like custom is nothing but self-interest. Positivists doubt that anything beyond consent assumes the force of law. In this paper, we adopt an experimental approach to test these claims. We show that the willingness to overcome a dilemma transcends self-interest. Cooperation is significantly higher in the presence of a meta-rule for the formation of customary law. Yet only if it is backed up by sanctions, law is significantly more effective than mere comity. Customary law guides behaviour into the normatively desired direction as normative expectations and behavioural patterns coevolve.