Do Jurors Give Appropriate Weight to Forensic Identification Evidence?
William C. Thompson
University of California, Irvine - Department of Criminology, Law and Society
Suzanne O. Kaasa
Northrop Grumman Technical Services/Defense Personnel Security Research Center
University of California, Irvine - Department of Psychology and Social Behavior
February 9, 2013
Journal of Empirical Legal Studies, Forthcoming
UC Irvine School of Law Research Paper No. 2013-124
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.
Number of Pages in PDF File: 67
Keywords: forensic science, jury, DNA, statistic, evidence, Bayes, probability, decision making, judgment
JEL Classification: C11, C44, C91, C92, K14, K41
Date posted: February 10, 2013 ; Last revised: July 25, 2013
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