Do Jurors Give Appropriate Weight to Forensic Identification Evidence?

Abstract

Do jurors give appropriate weight to forensic identification evidence? When judging the value of forensic evidence, are they sensitive to the probability of a false match? To answer these questions, we conducted two jury simulation experiments — the first with undergraduate participants, the second with members of a county jury pool. The experiments examined the weight that participants gave to forensic DNA evidence relative to Bayesian norms when evaluating a hypothetical criminal case. We found that aggregate judgments were generally consistent with Bayesian expectations, although people over-valued the DNA evidence when the probability of a false report of a match was high relative to the random match probability. Judgments of the chances the defendant was guilty varied appropriately in response to the variation in the probability of a false report of a match, as did verdicts. Our findings refute claims that jurors are always conservative Bayesians when evaluating forensic evidence and suggest, instead, that they use a variety of judgmental strategies and sometimes engage in fallacious statistical reasoning. In light of these findings, we identify circumstances in which forensic evidence may be over-utilized, discuss implications for legal policy, and suggest additional lines of research.

Keywords: forensic science, jury, DNA, statistic, evidence, Bayes, probability, decision making, judgment

JEL Classification: C11, C44, C91, C92, K14, K41