The Bayesian approach of forensic evidence evaluation: a necessary form of ‘survival’ of the ultimate issue rule

The ultimate issue rule has been widely criticised and has been abandoned in most common law jurisdictions. While its continuance is questioned in English criminal proceedings, this paper argues that there is scope for its survival in the context of forensic identification evidence. This paper refers to the standards for the formulation of evaluative forensic expert opinions referred to as ‘Bayesian approach’. The founding principles of this paradigm, initially associated with the successful use of DNA evidence in criminal trials, today are widely applied in many forensic identification disciplines. This paper highlights the major contribution this model makes by shaping up the role of the expert in a criminal trial to pronouncing on the weight of evidence and not to addressing the ultimate issue. By requiring the expert to evaluate the evidence and report on the probability of the findings under a set of propositions and leaving the determination of the probability of those propositions to the trier of fact, the Bayesian approach conforms not only to logic and reason but to the legal principles too.
Keywords: ultimate issue rule, DNA, individualisation, forensic identification evidence, Bayesian approach, prosecutor fallacy