Annex C - Previous research on not proven
Smithson and others (2007)
Smithson and others' study involved 104 mock jurors reading two trial scenarios - one criminal (murder) and one civil - alongside judicial instructions. Having done so, they were asked to return individual verdicts (rather than deliberating in jury groups). Half of the jurors were initially restricted to returning verdicts of guilty or not guilty, whilst the remainder had the further option of not proven. After completing initial questionnaires, jurors were asked to render their verdict again, but with the alternative set of options to that with which they were initially presented. Having analysed responses across the two conditions, the researchers concluded that "[c]ontrary to the hypothesis that the not proven verdict lures people away from convictions, we find in both trials that it lures them away from full acquittals to a greater extent". The reference here to 'full acquittals' is a reference to a not guilty verdict. As both not guilty and not proven verdicts are full acquittals in the Scottish system, movement from a guilty verdict to a not proven one might be regarded as rather more significant than movement from a not guilty verdict to a not proven one. In any event, the realism of the methods used in this study is particularly low and thus the reliance that can be placed on its findings is accordingly very limited.
Hope and others (2008)
A further exploration of the impact of a three-verdict system was undertaken by Hope and others, across two different studies. In the first study, 104 mock jurors were provided with a written summary of a sexual assault trial and jury directions. The mock jurors (who again returned verdicts individually rather than deliberating in groups) were randomly assigned to one of two experimental conditions: a two-verdict condition where they returned a verdict of either guilty or not guilty; and a three-verdict condition where they also had the option of a not proven verdict. The difference in acquittal rates was not statistically significant.
In the second study, Hope and others recruited participants to 28 juries (4 to 8 members). Jurors were provided with a written summary of a (non-sexual) assault trial and jury directions. There were three different versions of the trial, with the strength of the prosecution evidence varying (strong, moderate or weak). Half of the juries were restricted to returning a verdict of either guilty or not guilty whilst the rest also had the option of a not proven verdict. The juries were asked to reach a unanimous verdict if possible, and to record the verdict of the majority if not. Here, the research found that the proportion of jurors favouring conviction was higher where only two verdicts were available (35% rather than 22%, marginally statistically significant), but there was an association between the number of available verdicts and the outcomes (at juror level) only in respect of the "moderate" version of the evidential case.
Curley and others (2019)
Most recently, Curley and others have revisted the effect of the not proven verdict. In this study, 128 mock jurors (primarily student volunteers) listened to two audio vignettes of around six minutes long and were asked to return individual verdicts. Half of participants had the option of returning a not proven, not guilty or guilty verdict when listening to the first vignette, but were restricted to either not guilty or guilty when listening to the second. Meanwhile, the remainder had two verdicts available in the first vignette and three in the second. The difference in conviction rates was not statistically significant. Though it used audio vignettes rather than written summaries, the study shares the limited realism of its predecessors, and - aside from broader concerns about external validity - the fact that participants gave their verdicts under both the two and three-verdict conditions in quick succession may also have affected their verdict choices.
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