Date of Award

1-1-2001

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Master of Psychology

Faculty

Faculty of Community Services, Education and Social Sciences

First Supervisor

Associate Professor Alfred Allan

Abstract

Under the Criminal Code Compilation Act 1913 (WA), any number of individuals may be joined as co-defendants in a single trial, fanning a situation known as a joint trial. The charge/s against each defendant are considered separately and given a separate verdict by the jury. There is considerable debate in the legal arena as to the utility of joint trials, although to date little empirical research exists to substantiate any of the claims made. The present study aimed to contribute to the sparse knowledge base on joint trials by examining the impact of evidence strength on juror decision making in joint and single trials of the same defendant. Sixty mock juror university students were required to listen to an audiotaped trial summary about a hypothetical assault case that followed the same procedure as would be followed in Australian criminal courts. Evidence strength was manipulated so that defendant A had relatively weak and circumstantial evidence implicating him in the offence, and defendant B had very strong, substantive evidence implicating him in the offence. Two pilot studies confirmed that this manipulation was successful. The participants were assigned to one of three conditions - the single trial of defendant A, the single trial of defendant B, or the joint trial of defendants A and B. After listening to the trial summary, the participants were then required to give a verdict for the defendant/s, and rate the strength of the prosecution and defence evidence presented for the defendant/s. The hypothesis that the effect of joining their trials will be different for defendants A and B in terms of the proportion of guilty verdicts rendered for each defendant was supported. It was found that defendant A was significantly more likely to be found guilty in the joined condition than in the single condition (p < .05). There was no such effect observed for defendant B (p > .05). The second hypothesis that the effect of joining their trials will be different for defendants A and B on the perceived strength of prosecution evidence was also supported. Statistical testing revealed that there was a significant increase in the perceived strength of the prosecution evidence for defendant A in the joint condition, as c0mpared to the single condition (p < .05). There was no significant difference between the prosecution evidence strength ratings for defendant 8 in the single and joint conditions (p > .05). There was no support for the hypothesis that the effect of joining their trials will be different for defendants A and B on the perceived strength of defence evidence. For both defendants, there was no significant difference between defence evidence strength ratings in the joined and single conditions (p >.05). These results are interpreted with reference to impression formation theory. The limitations of the present study, including the sample, trial medium, trial elements, consequentiality of the task, and the trial materials are discussed. Directions for future research, such as improvements in the present study and additional sources of bias that may influence verdicts in joint trials, are also examined.

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