Probabilistic DNA evidence: the layperson’s interpretation

Abstract

A recent Australian High Court case held it was acceptable to express DNA results as either a frequency ratio or as an exclusion percentage. In order to understand if these two approaches could affect the outcome of a criminal trial, this study collected online survey data from the general public who were eligible for jury duty in Australia (n = 258). Participants were randomly assigned and completed two vignettes with two different forensic results that were manipulated in a 2×2 between-group design. Results found the way evidence was presented was sometimes statistically significant on the verdict in the case, and when not, the relationship was going in the predicted direction. Specifically when evidence was presented as an exclusion percentage, participants were more likely to convict than when presented with frequency ratio evidence. This is important as research suggests that once DNA evidence is admitted the effect can be difficult to undo, even with extensive cross-examination and testimony. DNA is a valuable tool for the criminal justice system; however, this study considers whether there is a need for standardisation in the way results are presented in a criminal trial to ensure jurors do not fallaciously reason about the evidence.

Keywords:

• DNA evidence
• jury decision-making
• exemplar cueing theory
• probabilistic reasoning
• forensic science

Acknowledgements

The author would like to thank Dr Dominique de Andrade, Dr Peter Cassematis, Mr Kerry Wimshurst and the anonymous reviewers for their comments on previous versions of this article.