The relationship between beliefs, strength of evidence, statistical presentation, and expert testimony on jury decision-making in DNA cases

According to the literature on jury decision-making, many factors account for how jurors reach a final verdict decision in criminal trials, including their pre-conceived beliefs about evidence. Given the recent popularity of DNA evidence, it is important to understand the factors that influence jurors' verdict decisions in criminal cases where DNA evidence is involved. Besides jurors' beliefs about DNA, other potentially important factors include strength of the evidence (SOE), expert testimony, and how DNA evidence is statistically presented. These factors were examined across two studies. Participants in Study 1 were 71 undergraduate university students who were selected to take part in a mock-jury study based on their responses to a questionnaire assessing their beliefs about DNA. After reading a transcript of a mock murder trial in which SOE was manipulated, jurors were required to rate various aspects of the case and to give a verdict rating. Results found no interaction between jurors' pre-existing beliefs about DNA and SOE. However, SOE impacted on jurors' verdict decisions with stronger DNA evidence leading to higher ratings of guilt than weaker DNA evidence. Participants in Study 2 were 228 undergraduate university students who agreed to take part in a mock jury study. They read a transcript of a mock murder trial manipulating SOE, expert testimony, and type of statistical presentation of DNA evidence. Jurors then rated various aspects of the case and gave a verdict rating. Results again found that when trial evidence was strong rather than weak, jurors gave higher guilt ratings. Furthermore, higher ratings of guilt, expert persuasiveness and usefulness of expert testimony were given by jurors who read DNA error rates presented as frequencies versus probabilities or ratios. There was also a significant interaction between type of expert testimony and statistical presentation. When expert testimony addressed how statistical information about DNA error rates can be presented in different, yet equivalent ways, jurors who read frequency error rates gave higher guilt ratings than those who read probability or ratio error rates. Overall, the results highlight the importance of ensuring that jurors are accurately educated about DNA evidence before entering the courtroom.