Abstract: The burden of proof is a central feature of all systems of adjudication, yet one that has
been subject to little normative analysis. This Article examines how strong evidence should
have to be in order to assign liability when the objective is to maximize social welfare. In basic
settings, there is a tradeoff between deterrence benefits and chilling costs, and the optimal proof
requirement is determined by factors that are almost entirely distinct from those underlying the
preponderance of the evidence rule and other traditional standards. As a consequence, these
familiar burden of proof rules have some surprising properties, as do alternative criteria that have
been advanced. The Article also considers how setting the proof burden interacts with other
features of legal system design: the determination of enforcement effort, the level of sanctions,
and the degree of accuracy of adjudication. It compares and contrasts a variety of legal
environments and methods of enforcement, explaining how the appropriate proof requirements
differ qualitatively across contexts. Most of the questions raised and answers presented differ in
kind—as well as in result—from those in prior literature.