The Structure of Evidence Law

Abstract

Here is a fairly straightforward sketch of the theoretical structure of evidence law. Fact-finding involves decision-making under uncertainty,1 that is, in situations where we cannot be absolutely sure of where the truth lies. The best that fact-finders can do, then, is assess the probability of liability. Fact-finding should be conducted rationally, therefore we can think of it as governed by the rules of probability theory.2 These rules provide a framework within which fact-finders should draw on their general experience to assess the probability of the evidence presented and, ultimately, of the facts in issue. Because fact-finders make decisions under uncertainty, evidence law needs to provide rules specifying the degree of certainty required for a verdict. Such rules are termed standards of proof. Standards of proof can only be set by considering what is at stake in a finding of liability, so at this point accounts of the rules governing civil and criminal trials diverge. In civil trials, the basic presumption is one of equality between claimant and defendant: a mistake affecting one side is as serious as a mistake affecting the other. Decision theory can then be used to show that the civil standard of proof should be set at a probability of 0.5, the ‘balance of probabilities’, a rule which has the additional merit of minimizing the number of expected fact-finding errors. The presumption of equality does not apply to criminal trials, where the censure involved in a finding of liability, as well as the ‘hard treatment’ which frequently follows such a finding, point to a higher standard of proof.3