A Likelihood Story: The Theory of Legal Fact-Finding

Abstract

Are racial stereotypes a proper basis for legal fact-finding? What about gender stereotypes, sincerely believed by the fact-finder and informed by the fact-finder’s life experience? What about population averages: if people of a certain gender, education level, and past criminal history exhibit a statistically greater incidence of violent behavior than the population overall, is this evidence that a given person within this class did act violently on a particular occasion?

The intuitive answer is that none of these feel like proper bases on which fact-finders should be deciding cases. But why not? Nothing in traditional probability or belief-based theories of fact-finding justifies excluding any of these inferences. Maybe intuition goes astray here. Or maybe something about the traditional theory of fact-finding is wrong. Arguing the latter, this article proposes a new theory of fact-finding. In contrast to historic probability and belief-based theories, this paper suggests that idealized fact-finding is an application of likelihood reasoning — the statistical analog of the ancient legal concept of the “weight of evidence” and the formal analog of modern descriptions of legal fact-finding as a process of comparing the relative plausibility of competing factual stories on the evidence.

This likelihood theory marks a fundamental change in our understanding of fact-finding, with equally fundamental implications for practice and procedure. The theory simplifies fact-finding, describing every burden of persuasion as an application of the same reasoning principle. It harmonizes recent scholarship on fact-finding, showing that work on the cognitive processes of fact-finders can be formalized in a comprehensive and coherent theory of the ideal fact-finding process. It explains evidentiary mores, justifying hostility to naked statistical evidence, for example. And it provides new insights into the effects of subjective beliefs on fact-finding, showing not only the harm that results from asking fact-finders to decide cases based on their personal beliefs about the facts, but also the way forward in reorienting fact-finding away from prejudice, bias, and subjective beliefs, and toward the firmer ground of the evidence itself.

Keywords: fact-finding; burden of persuasion; likelihood; probability; evidence

JEL Classification: K40, C25