Jury Understanding of DNA Evidence: An Empirical Assessment of Presentation Formats for Trace Evidence with a Relatively Small Random Match Probability

36 Pages Posted: 20 Dec 2003  

Dale A. Nance

Case Western Reserve University School of Law

Scott B. Morris

Illinois Institute of Technology - Institute of Psychology

Date Written: October 17, 2003

Abstract

In cases involving scientific evidence in the form of a test result linking the accused to a crime (e.g., DNA match), expert testimony sometimes can also provide a suitably reliable estimate of the chance of a coincidental match (the random match probability). Considerable controversy, however, attends the question of whether to allow testimony reporting that probability and, if so, in what form it should be given. Additional and related controversy concerns the implications of proficiency test results for testimony about the chance of false positive lab error, especially when that figure greatly exceeds the random match probability. This paper reports a large scale empirical study, using members of an Illinois jury pool, designed to contribute to our understanding of the issues involved. Our results confirm earlier research suggesting that jurors, rather than being credulously overwhelmed by the science, tend to undervalue forensic match evidence. On the other hand, our results differ from most prior research in showing that variation in the way the random match probability is presented and explained can reduce the extent of the undervaluation, without at the same time inviting inferential fallacies that would exaggerate the probative value of the match. And contrary to predictions, our results also show that incorporating information about comparatively large lab error rates, when it has any discernible effect, actually increases the jurors' assessed probability of guilt and willingness to convict.

Keywords: trace evidence, random match probability, DNA, prosecutor's fallacy, defense attorney's fallacy, inversion fallacy, misaggregation, Bayesian instruction, scientific evidence, lab error rates, vividness, averaging hypothesis, jury, cognitive error

JEL Classification: K41

Suggested Citation

Nance, Dale A. and Morris, Scott B., Jury Understanding of DNA Evidence: An Empirical Assessment of Presentation Formats for Trace Evidence with a Relatively Small Random Match Probability (October 17, 2003). Available at SSRN: https://ssrn.com/abstract=462880 or http://dx.doi.org/10.2139/ssrn.462880

Dale A. Nance (Contact Author)

Case Western Reserve University School of Law ( email )

11075 East Boulevard
Cleveland, OH 44106-7148
United States
216-368-3294 (Phone)
216-368-2086 (Fax)

Scott B. Morris

Illinois Institute of Technology - Institute of Psychology ( email )

3300 S. Federal Street
LS 248B, Main Campus
Chicago, IL 60616
United States
312-567-5932 (Phone)

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