Error Rates, Likelihood Ratios, and Jury Evaluation of Forensic Evidence.

Abstract

Forensic examiners regularly testify in criminal cases, informing the jurors whether crime scene evidence likely came from a source. In this study, we examine the impact of providing jurors with testimony further qualified by error rates and likelihood ratios, for expert testimony concerning two forensic disciplines: commonly used fingerprint comparison evidence and a novel technique involving voice comparison. Our method involved surveying mock jurors in Amazon Mechanical Turk (N=897 laypeople) using written testimony and judicial instructions. Participants were more skeptical of voice analysis and generated fewer "guilty" decisions than for fingerprint analysis (B=2.00, OR=7.06, p=<0.000). We found that error rate information most strongly decreased "guilty" votes relative to no qualifying information for participants who heard fingerprint evidence (but not those that heard voice analysis evidence; B=-1.16, OR=0.32, p=0.007). We also found that error rates and conclusion types led to a greater decrease on "guilty" votes for fingerprint evidence than voice evidence (B=1.44, OR=4.23, p=0.021). We conclude that these results suggest jurors adjust the weight placed on forensic evidence depending on their prior views about its reliability. Future research should develop testimony and judicial instructions that can better inform jurors of the strengths and limitations of forensic evidence.