Admissibility of forensic experts and evidence in criminal cases in England and Wales

Jurisdictions such as England and Wales operate under an adversarial justice system that has long grappled with wrongful convictions and miscarriages of justice, raising concerns about the reliability and regulation of forensic science. In criminal trials, judges and jurors frequently depend on forensic experts to interpret and communicate complex scientific and technical evidence. Research indicates that when faced with such complexity, factfinders may rely on descriptive decision-making models—such as peripheral cues—rather than normative models grounded in substantive evaluation. This tendency may contribute to the admission of unreliable expert evidence. Compounding this issue is the absence of a codified framework for assessing the admissibility of forensic expert testimony in English law, thereby engendering inconsistency in judicial assessments and affording courts wide discretion in their determinations.

This thesis addressed the following questions: (1) To what extent do the Court of Appeal (Criminal Division) adhere to normative legal standards when evaluating forensic expert evidence? (2) How do legal professionals and laypeople conceptualise expert credibility? (3) What factors influence lay assessments of expert credibility in trial-like settings?

To answer these questions, three empirical studies were conducted. Study 1 employed a mixed-methods analysis of 305 appellate judgments from the England and Wales Court of Appeal (Criminal Division), revealing inconsistent application of general admissibility criteria and limited engagement with scientific validity. Study 2 surveyed 231 legal professionals and UK-jury eligible laypeople, identifying divergence reasoning strategies: legal professionals relied on a context-driven but potentially misleading “intuitive checklist” while the lay participants drew on everyday heuristics. Study 3 adopted a between-subjects experimental design (N = 376) to examine how expert testimony communication (intuitive; quasirational; analytical) and domain expertise (DNA; Digital) influence lay credibility assessments. DNA experts were rated higher in substantive credibility, but communication style significantly shaped perceptions: intuitive delivery enhanced likeability, while analytical delivery increased perceived substantive credibility. Notably, analytical presentation reduced the credibility gap between DNA and digital experts, suggesting that delivery can outweigh disciplinary prestige.

Grounded in an integrative theoretical framework that synthesises cognitive continuum theory, bounded rationality, and evidentiary jurisprudence, this thesis offers a novel interdisciplinary contribution to the study of forensic science in legal contexts. It proposes an evaluative model that bridges psychological insights with legal standards to improve the assessment of expert evidence. The findings have implications for judicial training, expert witness guidelines, and policy reform aimed at enhancing the integrity of legal proceedings.