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

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.