The Prosecutor's Fallacy and Expert Testimony: A Modern Take Using Likelihood Ratios

Abstract: Forensic examiners and attorneys need to know how to express evidence in favor or against a prosecutor's hypothesis in a way that avoids the prosecutor's fallacy and follows the modern reporting standards for forensic evidence. This article delves into the inherent conflict between legal and scientific principles, exacerbated by the prevalence of alternative facts in contemporary discourse. Courts grapple with contradictory expert testimonies, leading to a surge in erroneous rulings based on flawed amicus briefs and testimonies, notably the persistent prosecutor's fallacy. The piece underscores the necessity for legal practitioners to navigate this fallacy within the modern forensic science framework, emphasizing the importance of reporting likelihood ratios (LRs) over posterior probabilities. Recognizing the challenge of lay comprehension of LRs, the article calls for updated recommendations to mitigate the prosecutor's fallacy. Its contribution lies in providing a detailed analysis of the fallacy using LRs and advocating for a sound interpretation of evidence. Illustrated through a modified real case, this article serves as a valuable guide for legal professionals, offering insights into avoiding fallacious reasoning in forensic evidence assessment.