NCJ Number
196029
Journal
Canadian Society of Forensic Science Journal Volume: 35 Issue: 2 Dated: June 2002 Pages: 77-90
Date Published
June 2002
Length
14 pages
Annotation
When presenting forensic DNA match evidence during a trial, forensic scientists may use a variety of probabilistic statements to convey the probative value or significance of such a match; the current study compared mock jurors' responses to three statements used in this context.
Abstract
Probabilistic statements regarding the significance of a DNA match may include posterior probability statements, qualitative statements, likelihood ratios, profile incidence rates, random match probabilities, and source attribution statements. In the legal community, there has been a great deal of discussion and legal argument regarding the admissibility of the DNA profiling technique, the standardization of laboratory procedures, the compilation and use of population databases, and the significance of a DNA match; however, the legal community has, for the most part, deferred to forensic scientists regarding the manner in which the significance of a match is articulated. Among forensic scientists, there has also been debate about the calculation and communication of these probabilities. Little attention has been given, however, to the more fundamental issue of how these statements are interpreted by the triers of fact. Participants in the current study were 336 undergraduate psychology students at York University in Toronto, Ontario, and 33 undergraduate psychology students from Ryerson Polytechnic University in Toronto. The facts of the scenario were compiled from a number of actual cases that had been tried in Canada. All of the evidence was circumstantial. The independent variable was the statement regarding a DNA match; the significance of the DNA match was phrased in one of three ways: as a frequency of occurrence of the DNA profile, the random match probability of the DNA profile, or a statement attributing the scene sample to the defendant on the basis of the DNA match (source attribution statement). The influence of presentation style (reading versus listening) on judgements of guilt was also examined. The study found no statistically significant differences in guilt ratings when frequency of occurrence, random match probability, and source attribution statements were compared. Reading and listening conditions also produced no statistically significant differences in guilt ratings, although a trend approaching significance may be of interest for further research. Female participants had significantly higher guilt ratings overall, although male participants produced ratings of greater variability. The study concluded that variations in the statements used by forensic scientists to assign weight to a DNA match did not significantly influence assessments of guilt by the mock jurors, regardless of presentation method. 2 figures, 31 references, and an appended mock jury questionnaire