Painting the target around the matching profile: the Texas sharpshooter fallacy in forensic DNA interpretation

Abstract

Forensic DNA analysts tend to underestimate the frequency of matching profiles (and overestimate likelihood ratios) by shifting the purported criteria for a ‘match’ or ‘inclusion’ after the profile of a suspect becomes known-a process analogous to the well-known Texas sharpshooter fallacy. Using examples from casework, informal and naturalistic experiments, and analysts’ own testimony, this article demonstrates how post hoc target shifting occurs and how it can distort the frequency and likelihood ratio statistics used to characterize DNA matches, making matches appear more probative than they actually are. It concludes by calling for broader adoption of more rigorous analytical procedures, such as sequential unmasking, that can reduce the sharpshooter fallacy by fixing the target before the shots are taken.