The Ultimate Opinion Rule and Forensic Science Identification

Abstract

For decades, scientists, statisticians, psychologists, and lawyers have urged forensic scientists who compare handwriting, fingerprints, fibers, shoeprints, tool¬marks, paint chips, and other items to change their ways. At last it seems that “[t]he traditional assumption that items like [these] have unique patterns that allow experts to accurately determine their source . . . is being replaced by a new logic of forensic reporting.” With the dissemination of “probabilistic genotyping” software that generates “likelihood ratios” for DNA evidence, the logic is making its way into U.S. courts. Recently, however, the Justice Department’s Senior Advisor on Forensic Science commented on a connection between prominent statements endorsing the new logic and an old rule of evidence concerning “ultimate issues.” He suggested that “some people . . . would like to pretend that [Federal Rule of Evidence 704] doesn't exist, but it actually goes against that school of thought.” This essay considers the nexus between Rule 704 and forensic-science testimony, old and new. It concludes that the rule does not apply to all source attributions, and even when it does, it supplies no affirmative reason to admit them over likelihood statements. In ruling on objections to traditional source attributions, courts would be remiss to think that Rule 704 favors this type of opinion testimony over the newer evidence-centric statements.

Keywords: evidence, forensic science, identification, expert opinions, ultimate issues, Rule 704, likelihoods, individualization, probability