Understanding Jurors Use of Highly Diagnostic Statistical Evidence

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Understanding and Improving Jurors' Use of Highly Diagnostic Statistical EvidenceJonathan J. KoehlerThere is no more pressing issue in the controversy that swirls around the use of scientific evidence in the courtroom than how jurors interpret and use the tiny statistics that arise in connection with forensic science evidence. For example, many people assume that these statistics are persuasive in cases where a criminal suspect matches traces of DNA evidence recovered from the scene of a violent crime. That is, they believe that after hearing coincidental match statistics that range from 1 in thousands to 1 in billions, jurors will be convinced that the matching suspects are indeed the source of the recovered DNA evidence. Others believe that jurors will be confused by the numbers and will not find the evidence compelling.Who's right? My own preliminary research with mock jurors indicates that the way in which statistical evidence is presented may exert a powerful effect on how legal decision makers think about and use the evidence. I offer a descriptive theory and two experiments to help understand howlegal decision makers respond to highly diagnostic statistical evidence.The theory -- called exemplar cueing theory -- predicts when decisionmakers will find statistical match evidence relatively strong andpersuasive and when not. It holds that judgments and choices related tothe strength and persuasiveness of statistical match evidence are guided bythe availability of coincidental match exemplars. To the extent decisionmakers can easily generate examples of chance matches, the evidence willseem less compelling as proof of identity. This theoretical framework is broad enough for understanding how peoplethink about many low probability events such as one's chances of winningthe lottery or the risks posed by environmental catastrophe or geneticdisease. It is also sufficiently refined that it may help us understandwhen jurors will be persuaded by a defendant's argument that a DNA match ispurely coincidental.These data will be useful to a variety of constituencies, from the judgesand policy makers who must develop instructions and standards for theadmissibility of statistical evidence, to the scientists, law enforcementofficers, and attorneys who must find ways to communicate the meaning of amatch.