The Persuasive Powers of DNA: An Experimental Study in Perceptions of Expert Evidence

Abstract

Abstract-This article presents the results of an experimental study where mock-jurors were tasked with interpreting the presentation of DNA evidence. The 200 university student participants were exposed to one of five murder scenarios where the information about the DNA evidence was manipulated. The results showed that participants were more likely to convict when the DNA match statistic was presented as a probability (0.1%) and focused on the defendant, less likely to convict when it was presented as a frequency (1 in 1,000) and focused on a broader reference group, and even less likely in the control scenario with no DNA evidence. The forensic knowledge of participants was also explored, and more than three-quarters demonstrated reasonable understanding of the individuating capacity of DNA evidence. Participants recognized that while DNA has the capacity to determine guilt, it is insufficient on its own to convict or acquit. The implications for the presentation of expert testimony and judicial instruction are canvassed, and the broader ramifications for the education of jurors and legal personnel are discussed.Index Terms-DNA evidence, expert evidence, juror decisionmaking, match statistics I. INTRODUCTION HE forensic application of DNA has increased the frequency of criminal investigations that rely on biological material for identification purposes (Goodman-Delahunty & Tait, 2006) and this impact clearly carries through into the adjudicative setting (Amorim, 2012;Briody, 2003;. Due to its use in violent and high profile cases, and its role in overturning wrongful convictions, DNA analysis has been considered the great panacea for criminal investigations and court proceedings (Butler, 2001;Cooper, 2012; Kirby, 2010;Lincoln & Wilson, 2005;Lynch, 2013). It was once characterized in a It is, of course, difficult to argue against the power of DNA to incriminate the guilty, or deny its crucial role in exculpating the innocent, but like all powerful tools it has the capacity to produce undesirable consequences (Edmond, 2011; GoodmanDelahunty & Tait, 2006; Kirby, 2010;Langdon & Wilson, 2005;Lynch, 2013;McCartney, 2006). In the wake of the claim last century that little is known "about how laypersons respond to DNA evidence" (National Research Council, 1996), the body of research in this area has expanded significantly, providing knowledge about the weight that legal decision makers assign to it in forensic settings (Briody, 2003; GoodmanDelahunty & Hewson, 2010;Goodman-Delahunty & Wakabayashi, 2012;Findlay, 2008;Koehler, 2001;Koehler & Macchi, 2004;Kruse, 2012;Lynch, 2013;Smith & Bull, 2012;Smith, et. al., 2011;Wheate, 2006;.Studies in Australia demonstrate that there is potential to overly rely on DNA (Briody, 2003;. In one jurisdiction, it was found that cases of murder and sexual assault that include DNA have a higher likelihood of going to trial and a higher likelihood of securing a conviction than do cases without this type of evidence (Briody, 2003;. Such empirical studies raise serious questions about how ju...