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

In cases involving scientific evidence linking the accused to a crime (a “match”), expert testimony sometimes can provide a suitably reliable estimate of the chance of a coincidental match. Controversy attends the question whether, and in what form, to allow testimony reporting that probability. Further controversy concerns the implications of laboratory proficiency tests for the presentation of testimony about the chance of lab error. This large-scale empirical study, using members of an Illinois jury pool, confirms earlier research suggesting that, contrary to some predictions, jurors tend to undervalue forensic match evidence. Our results differ from most prior research, however, in showing that variation in the way the random-match probability is presented and explained can reduce the undervaluation, that it can do so without inducing significant inferential fallacies, and that incorporating information about comparatively large lab error rates, when it has any discernible effect, increases jurors' willingness to convict.