Date of Award


Degree Type


Degree Name

Master of Psychology


Faculty of Community Services, Education and Social Sciences

First Advisor

Dr. Alfred Allan


When two or more people are alleged to have committed a crime together they are automatically tried together in a joint trial. Defendants can apply to have a joint trial severed into separate trials, but they are rarely granted. However, joint trials might be biasing against defendants in that they might have a greater likelihood of obtaining aguilty verdict than if they had separate trials. A review of the literature indicated that authors have several hypotheses why joint trials might be biasing, though there is no conclusive evidence that this is the case. This study used a mock juror paradigm to investigate whether joint trials are biasing toward defendants. Results indicated that there was not a significant difference in the proportion of guilty/not-guilty verdicts across one, two, and three defendant trial conditions when all defendants involved were charged with the same offence (assault). It was concluded that having multiple defendants in a trial was not, on its own, biasing against the first or second defendants. However, having a co-defendant with a more serious charge (grievous bodily harm) led to a significant increase in the proportion of guilty verdicts assigned to the defendant in two out of three scenarios. It was concluded that it is possible that paired with a co-defendant who has a more serious charge is biasing, but future research to assess this factor more thoroughly to make firmer conclusions. Therefore, it might be important for judges to consider differences in charge seriousness when deciding whether to grant separate trials to defendants who apply for them. It is suggested that there was little information for participants in this study to remember and organise, compared to the amount of information presented in real trials. Further, damaging inadmissible evidence against defendants was not included in this study. Future research should investigate the effects of length and complexity, and inadmissible evidence to assess for any biases against joint trial defendants.