Expert Evidence in Criminal Law: Rethinking Reliability in England and Wales after the Lucy Letby Case

Abstract

High-profile criminal cases increasingly depend on complex scientific and medical evidence, yet the legal framework governing its admissibility in England and Wales remains strikingly underdeveloped. This article examines that tension through the case of Lucy Letby, a neonatal nurse convicted of the murder of seven infants and the attempted murder of seven others at the Countess of Chester Hospital between 2015 and 2016, where the prosecution case relied heavily on expert interpretation of clinical events. It argues that the current admissibility regime, rooted in common law principles and supplemented by procedural rules, remains permissive and lacks sufficiently robust mechanisms to rigorously evaluate the reliability of expert testimony. Situating these concerns within the Law Commission’s 2011 proposals and comparative experience under the United States Daubert standard, the article contends that neither procedural reform nor formal gatekeeping tests have resolved the underlying epistemic tensions between law and science. The problem is structural: the criminal process lacks the epistemic infrastructure necessary to evaluate contested scientific evidence in a principled and consistent manner. In response, the article advances a set of integrated reforms, including more structured judicial reasoning, a proportionate pre-trial reliability review mechanism, clearer expectations around accreditation and continuing expertise and enhanced judicial engagement with scientific reasoning. Ultimately, it argues for a shift towards “epistemic literacy” as the foundation for a more transparent and defensible approach to expert evidence.