The influence of validation data on logical and scientific interpretations of forensic expert opinions

Abstract Forensic expert opinions profoundly influence legal outcomes, yet how judges, jurors, and lawyers should interpret these opinions and what information helps them do so remains underexplored. We highlight the Bayesian solution provided by Morris (1971, 1974, 1977). Rather than adopting expert assessments at face value, recipients assign weight to expert opinions through their own uncertainties regarding what opinions experts would provide under each considered proposition. This “performance uncertainty” is distinct from recipients’ uncertainty about which proposition is true. Validation data reduce performance uncertainty, enabling recipients to weight expert opinions based on demonstrated performance. We illustrate this framework through examples spanning categorical conclusions, likelihood ratios, ranges, and multiple experts, demonstrating how it accommodates case-specific factors, incomplete information, and varying recipient beliefs. Though recipients will not conduct explicit computations, the key implication of Bayesian reasoning remains that, regardless of an expert’s opinion scale, judicial stakeholders require access to detailed performance data to make scientifically defensible interpretations of expert opinions. Restricted access to validation data prevents recipients from updating their performance beliefs with empirical evidence, leaving interpretations dependent on whatever initial assumptions each recipient brings. Science in forensics requires not just generating validation data but ensuring meaningful access for those interpreting expert opinions.