Probability Arguments in Criminal Law - Illustrated by the Case of Lucia de Berk

Herman Philipse

doi:10.18352/ulr.310

Abstract

Which logic of probability should be applied with regard to factual hypotheses in criminal cases? In this article, I discuss two possible logical reconstructions of the so-called Coincidence Argument, which played a crucial role in the conviction of Lucia de Berk by the Court of Appeal of The Hague (Gerechtshof ’s-Gravenhage) in 2004. If the argument is construed as an instance of the Law of Likelihood, nothing follows with regard to the probability that Lucia was a serial killer. If, however, the Argument from Coincidence may be interpreted charitably as an instance of Bayesian updating, the Court of The Hague did not fathom the diversity of the data needed in order to make it sound. Clearly, the Court had an insufficient grasp of the logic involved in the Coincidence Argument. Since this example is not atypical, I recommend law faculties to include probability logic (inter alia) in their courses on legal reasoning.

Highlights

Courses in Legal Reasoning offered by law faculties typically restrict the topic to forms of argument used in the application of legal rules
Must not instruction in legal reasoning include the many types of argument used in legislative drafting, for example? And what about the inquisitorial task of Continental criminal judges to establish the facts of the case before applying criminal law, as specified, for example, by the Dutch Code of Criminal Procedure (Wetboek van Strafvordering), Article 338 et seq.? Judges cannot entirely leave this task to scientific experts, since they have to understand and evaluate the experts’ arguments
I focus on one kind of reasoning that is central to this latter endeavour: probability arguments concerning factual hypotheses

Summary

Prelude

Courses in Legal Reasoning offered by law faculties typically restrict the topic to forms of argument used in the application of legal rules. With regard to most criminal cases, the evidence only makes it probable to a certain extent that the defendant is, or is not, guilty. It follows that probability arguments nearly always play a crucial role. Many experts have recommended Bayesianism with regard to cases such as Lucia de Berk’s Using this well-known case as an illustration, I argue that a Bayesian approach triggers some new questions, at least one of which I. Probability Arguments in Criminal Law – Illustrated by the Case of Lucia de Berk have not found in the existing literature. I conclude that a more comprehensive course on legal reasoning should be obligatory for law students (Section 5).[1]

A famous Dutch legal error

The Coincidence Argument as a likelihood comparison

A Bayesian version of the Coincidence Argument

Conclusion