Expert Evidence

Abstract

Abstract Chapter 11 is concerned with expert evidence, a topic traditionally conceptualized as an exceptional category of ‘opinion evidence’ and frequently marginalized in Evidence textbooks. Our approach emphasizes institutional context and epistemological foundations, and centrally features forensic science evidence—which is increasingly influential in modern criminal litigation (and sometimes associated with miscarriages of justice). The supposed dichotomy between ‘facts’ and ‘opinions’ is conceptually dubious but rarely significant in practice. It is more illuminating to think about expert evidence in terms of the epistemic warrant it supplies for criminal verdicts. Building on Ron Allen’s perceptive distinction between fact-finder ‘education’ and ‘deference’, a model of rational reliance on forensic expertise is proposed and tested against doctrinal precedents and principles. Expert witnesses must be competent and objective. However, these elementary qualifications are complicated in practice by the adversarial structure of criminal litigation. Traditional mechanisms of evidence-testing, including cross-examination, are not well-suited to exposing the deficiencies of expert evidence. If radical solutions, such as court-appointed experts, flatter to deceive, more modest procedural reform may increase institutional resilience, viewed in the broader context of criminal proceedings as a whole. Regulatory structures, codes of practice, professional ethics, and pre-trial safeguards might better enhance the quality of forensic science evidence than trial-centric measures. As a doctrinal matter, admissibility of expert evidence essentially boils down to competence (Davie), relevance and ‘helpfulness’ (Turner). Other evidentiary rules are either obsolete (ultimate issues) or fact-specific applications of the general Turner standard. More recently, there is (equivocal) doctrinal flirtation with a supplementary ‘reliability’ criterion, especially in relation to novel forensic science or new technological applications. Further insight is derived from comparative experiences, especially US Daubert jurisprudence and major reports (NRC, PCAST) criticizing the methodological underpinnings of forensic science. In England and Wales, legislative policies animating the Law Commission’s unimplemented Expert Evidence Bill have been partly incorporated into Part 19 of the Criminal Procedure Rules and Practice Directions.