Edinburgh Napier University

  The current thesis aimed to identify the process through which jurors reach their decisions, and to investigate the factors that may make the trial by jury process unfair for the individuals involved in criminal trials (i.e., the defendant, the prosecution and the defence). An initial literature review highlighted that two separate threshold models may be able to explain the processes (i.e., both rational and intuitive) through which jurors reach their decisions: 1) the Diffusion Threshold Model and 2) the Counter Threshold Model. The first study investigated which model of juror decision making was appropriate. In this quasi-experiment, 60 participants took part, and made verdicts (of Guilty, Not Guilty and Not Proven) over nine vignettes. Participants were asked to rate the evidence as either Guilty, Not Guilty or Not Proven, and to state how likely (from 1-100) they thought it was that the defendant was guilty. After all the evidence had been presented, participants were asked to give a verdict. Then, participants were asked to state the last piece of evidence they needed to reach said verdict (symbolising the threshold). The results suggested that the Diffusion Threshold Model best explained the decision processes of jurors. The second study investigated if the reaching of a threshold caused confirmation bias and/or evidence distortion to occur, and if information interaction allowed said threshold to be reached. Each of the 108 participants listened to one vignette, which contained an opening statement, eight pieces of evidence (four prosecution and four defence) and two closing statements (one prosecution and one defence). Participants were asked to state their perceived likelihood of guilt in regard to the defendant after each piece of evidence. After all the evidence had been presented, participants were asked to give a verdict and to state the last piece of evidence they needed to give said verdict (symbolising the threshold). The results showed that information integration occurred throughout a trial, but the reaching of a threshold promoted confirmation bias. The third study looked to investigate potential factors that may have an impact on juror perceptions of guilt and the verdicts given by jurors. In this study, 128 participants listened to two vignettes. The vignettes were manipulated, and counterbalanced, for both the anchor (strong versus weak initial piece of evidence) and number of verdicts available (two-verdicts were available or three-verdicts were available with the additional Not Proven verdict) factors. Participants also completed the Pre-trial juror bias questionnaire (PJAQ), which allowed participants to be categorised according to their bias. The results highlighted that pre-trial biases did have an impact on the belief of guilt given, and that jurors in a three-verdict system were less likely to give a Not Guilty verdict in comparison to jurors in a two-verdict system. The findings in the current thesis have suggested that the Diffusion Threshold Model can adequately explain the process through which jurors reach their decisions, that individuals integrate information until they reach an appropriate threshold, that the reaching of a threshold can promote confirmation bias, and that extra-legal factors, such as pre-trial biases, can have an impact on juror perceptions of guilt. The results also highlighted that certain factors (i.e., thresholds that promote confirmation bias) may make the trial by jury process unfair for individuals (i.e., the defendant) who are involved in a criminal trial.