The present paper combines legal history with economic theory so to explain the passage from custom to law. Economists have usually explained the shift from customary to statutory law (that is, from spontaneous to formal rules) either in terms contractualism or evolutionism. In the first case, law is the only efficient solution for a Hobbesian-like immanent social conflict. In the second case, customs do create an efficient enough equilibrium. Law comes on a later stage just to formalise an already accepted rule, vesting the custom with a formal status. Neither theory, however, is fully able to explain the transition from custom to law. One struggles with the very acceptance of customs in the first place. The other fails to provide a satisfactorily explanation of the passage from custom to law. The present work seeks to reconcile the two theories by looking at the economic advantages of statutory law over custom. Unlike the first theory, it does not deny that customs may produce a relatively efficient status, but it seeks to explain why, at a certain point, customs were considered as inadequate and statutory law became more desirable. Our answer lies in the publication of written rules, for the presumption of knowledge it entails. Presumption of knowledge of the applicable rules is one of the elements that (oral) customs could not provide to contracts. Although somewhat neglected in many studies on customs and legislation, publication is a crucial element for our understanding of the passage from spontaneous custom to positive law. The work shall first introduce the passage from customary to statutory law in both legal and economic theories. Then, it will analyse the deep symmetry between the number of agents involved and the number of transactions on the one hand and the progressive replacement of customs with statutes on the other. The conclusions of such an analysis will be used to prove the crucial role played by the presumption of knowledge, which is perhaps the missing link between different economic theories on customs and law.
