Scottish jury research: findings from a mock jury study

Executive summary

This report presents the findings of a large-scale mock jury study in Scotland. The Scottish jury system differs from most English language jurisdictions in three main ways:

• There are three verdicts: guilty, not guilty and not proven (most major English language jurisdictions only have two verdicts: guilty and not guilty).
• Each jury has 15 members (rather than the typical 12).
• Verdicts are returned by a simple majority (eight out of 15 jurors, rather than requiring juries to reach unanimity or near unanimity).

The Scottish Government commissioned this study to address two overarching questions:

• What effects do the unique features of the Scottish jury system have on jury reasoning and jury decision making?
• What are jurors' understandings of the not proven verdict and why might they choose this verdict over another verdict?

Research methods

This study is the largest of its kind ever undertaken in the UK, involving 64 mock juries and 969 individual participants. The research team staged jury deliberations between May and September 2018, in venues in central Glasgow and Edinburgh. Jurors were recruited to be broadly representative of the Scottish population aged 18-75 in terms of gender, age, education and working status. This meant that the mock juries were similar in demographic composition to the actual population eligible for jury service. In order to assess the effect of the Scottish jury system's unique features on decision-making, juries varied in terms of the number of verdicts available to them (two or three), jury size (12 or 15) and the size of majority they were required to reach (simple majority or unanimity).

Each jury watched a video of either a mock rape trial or a mock assault trial, lasting approximately one hour. Short clips from the two trials are available to watch online; links are in the footnote below.^[3] Jurors completed a brief questionnaire recording their initial views on the verdict, before deliberating as a group for up to 90 minutes and returning a verdict (if the jury had been able to arrive at one). After returning their verdict, jurors completed a final questionnaire covering their beliefs about the not proven verdict and views about the deliberation process, as well as their final views on the verdict.

The data generated from the study included: quantitative data from the questionnaires; quantitative and qualitative data from the deliberations (which were filmed, audio recorded and analysed by the researchers); and quantitative 'metadata' on each jury (e.g. length of deliberations, verdict returned, etc).

Realism and study limitations

In comparison with many previous mock jury studies, this research was significantly more realistic. It used relatively long filmed trials rather than relying on transcripts, with the trials reviewed for realism by senior Scottish legal practitioners. It included a lengthy group discussion element (some studies either omit this or allow very limited time). Each mock jury included either 12 or 15 people (smaller groups are common in much mock jury research).

Nonetheless, it is important to bear in mind the limitations of any research involving mock juries. First, participants knew that they were not acting as jurors in a real trial. The vast majority of participants appeared to take the exercise seriously - as indicated by the fact that discussions between jurors who disagreed on the verdict regularly became animated. However, it is not possible to control for any impact that the artificial nature of the mock jury experience might have had on deliberations or verdicts.

Second, though the study was the largest of its type to date in the UK, the total number of juries (64) was nonetheless relatively small, making it unlikely that anything other than large differences in verdicts between juries would be picked up statistically. However, the 64 juries were made up of 863 individual jurors, each of whom was asked for their individual view on what verdict should be returned (using questionnaires).^[4] This much larger sample of jurors means we can compare jurors asked to choose between two and three verdicts, those asked to reach a simple majority and those asked to reach a unanimous verdict, and jurors on 12 and 15-person juries.

This data can tell us whether or not particular features of the jury system might incline jurors in one direction or another (for example, whether being required to reach a unanimous decision might make a juror more or less likely to support an acquittal). As a result the research may indicate whether changes to the jury system, if applied across a larger number of finely balanced trials, might affect the likelihood of juries returning a particular verdict. However, we cannot use this data to arrive at an estimate of how many more juries might return a particular verdict.

Third, the findings in this study are based on jurors' responses to two specific trials, both of which were deliberately finely balanced in order to encourage discussion of the not proven verdict. Had the evidence in these trials been differently balanced, or had we used different kinds of cases, the balance between verdicts would probably have been different.

Important note on the findings

As expected given the sample size, there were no statistically significant differences in the number of guilty versus acquittal verdicts returned between 12 and 15-person juries, two-verdict and three-verdict juries, or between juries asked to reach a simple majority and those asked to reach a unanimous verdict.

However, there were a number of significant differences in the verdicts favoured by individual jurors. Therefore the key findings section below focuses primarily on what difference the unique features of the Scottish system make to individual jurors' verdict preferences, and to the process by which they reach their verdicts.

Key findings

The overarching finding is that juror verdicts were affected by how the jury system was constructed. The research found that the number of jurors, the number of verdicts available, and the size of majority required do have an effect on verdict choice.

In other words, jurors' verdict preferences, in finely balanced trials, are not simply a reflection of their assessment of the evidence presented, but can also be affected by features of the jury system within which this evidence is considered.

What difference does the size of the jury make?

What difference does jury size make to verdict choice?

Jurors in 15-person juries were less likely to change their minds on the verdict than people in 12-person juries.

Jurors in 15-person juries were more likely than jurors in 12-person juries to think the verdict should be guilty (after deliberating). However, this does not necessarily indicate that 15-person juries would be more likely to return guilty verdicts across a larger number of differently balanced trials. Rather, it may reflect the fact that, where a 15-person jury is split, more people (on average) need to change their position to facilitate a verdict than would be the case for a 12-person jury. This, in turn, may mean that 'minority' jurors in a 15-person jury have less motivation to shift their position to bring deliberations to a close. In this study, those supporting a guilty verdict were generally in the minority, but this will not always be the case. This finding may, therefore, indicate that individuals on larger juries are more likely to stick with their initial view, rather than indicating a greater propensity for larger juries to convict. This is supported by the finding that jurors in 15-person juries were less likely than jurors in 12-person juries to change their mind about the verdict.

What difference does jury size make to how juries reach their verdicts?

15-person juries were associated with somewhat lower levels of juror participation than 12-person juries across a number of measures.

For example, in 15-person juries: jurors were more likely to be observed wanting to contribute, but being unable to do so; there were more dominant jurors and more minimally contributing jurors; and jurors gave lower ratings of their own influence over the verdict.

However, the higher participation level in 12-person juries was not associated with longer deliberations: there was no difference in average deliberation length between 12-person and 15-person juries. Similarly, there was no difference between 12 and 15-person juries in the number of evidential issues discussed, or the extent or accuracy of discussion of legal issues.

Potential implications of reducing jury size

Taken together, these findings suggest that reducing the number of jurors on Scottish juries from 15 to 12:

• Might lead to more jurors participating more fully in the deliberations.
• Would be unlikely to have much impact on deliberation length or the range of evidential or legal issues discussed.

What difference does the size of the majority required make?

What difference does the majority required make to verdict choice?

Jurors who were asked to reach a simple majority were more likely to favour a guilty verdict than jurors asked to reach a unanimous verdict.

However, this does not necessarily mean that requiring jurors to reach a unanimous verdict would result in fewer guilty verdicts. It might simply reflect a greater tendency for jurors to change from agreeing with the minority view to agreeing with the majority view (in 51 out of 64 juries, the majority wanted to acquit the accused at the start of deliberations). However, there is some evidence to suggest that jurors who are in the minority at the start of deliberations may be less willing to shift their view towards a majority preference for guilty than they would towards a majority preference for acquittal - five of the six unanimity juries that started with a majority for guilty could not reach a unanimous decision and ended up hanging, compared with only one of the 25 unanimity juries that started with a majority for acquittal.

What difference does the size of the majority required make to how juries reach their verdicts?

Jurors asked to reach a unanimous verdict took substantially longer to deliberate than did those required to reach a simple majority.

Although there were no significant differences between unanimity juries and simple majority juries in observed levels of juror participation, there were some differences in jurors' own perceptions of their involvement. In particular, jurors asked to reach a unanimous verdict were a little more likely to feel they had been fully involved and had influenced the jury's decision.

In combination with the longer average deliberation time for unanimity juries, this suggests that requiring juries to reach a unanimous verdict may provide greater opportunity for everyone to feel that they have been able to put their views across before a verdict is reached. It is worth noting, however, that this was not associated with any increase in the range of evidential issues discussed or the extent or accuracy of discussion of legal issues.

Potential implications of changing the size of the majority required to reach a verdict

Taken together, these findings suggest that changing from the current simple majority system to unanimity:

• Might incline more jurors towards an acquittal - and might, therefore, lead to more acquittals over a larger number of trials. As noted above, however, it is not possible to estimate the likely scale of any such impact, since the effect will vary depending on factors including the balance of evidence and the initial balance of opinion in the jury in each trial.
• May lead to longer deliberations and more jurors feeling that they have been involved in the deliberations (although there is no evidence this would increase the range of evidential or legal issues discussed).

What difference does the not proven verdict make?

What difference does the not proven verdict make to verdict choice?

Where the not proven verdict was available, acquitting juries tended to choose not proven rather than not guilty as the means to acquit the accused. Individual jurors were also less likely to favour a guilty verdict when the not proven verdict was available.

26 out of 32 juries where not proven was available returned acquittals and, of those 26, 24 returned not proven verdicts and only two returned not guilty verdicts. This suggests that, in finely balanced trials, juries have a preference for acquitting via not proven rather than not guilty.

Individual jurors were significantly less likely to favour a guilty verdict when the not proven verdict was available. This difference was apparent both before and after deliberating, indicating that the availability of not proven has an effect on individual verdict preferences independent of any impact of group deliberation.

What difference does the not proven verdict make to how juries reach their verdict?

The presence or absence of the not proven verdict had little impact on the length of deliberations; the number of evidential issues raised; the extent or accuracy of the discussion of legal issues (other than discussion of the not proven verdict itself); or on levels of juror participation across various measures. However, its availability was associated with slightly lower levels of dissatisfaction (although not with significantly higher levels of satisfaction) with the experience of serving on a jury.

Potential implications of removing the not proven verdict

Taken together, these findings suggest that removing the not proven verdict:

• Might lead to more jurors favouring a guilty verdict, which might, therefore, lead to more guilty verdicts over a larger number of trials. As noted above, however, it is not possible to estimate the likely scale of any such impact, since the effect will vary depending on factors including the balance of evidence and the initial balance of opinion in the jury in each trial.
• May not have much impact on other key aspects of the jury decision-making process, such as deliberation length or juror participation.
• May be associated with a slight increase in juror dissatisfaction.

Were there any differences in the impact of jury size, majority required and the number of verdicts between the rape and assault trial?

In both the rape and the assault trials, requiring a unanimous verdict was associated with more jurors favouring acquittal after deliberation.

The general pattern of differences in individual jurors' verdicts by both jury size and number of verdicts, reported above, was also similar across both trial types - in other words, individual jurors were less likely to shift their views in 15-person than in 12-person juries, and less likely to favour guilty when the not proven verdict was available. However, these differences were only statistically significant for the assault trial. Findings for individual trial types are based on around half the sample - 430 jurors. This means that bigger differences would be required to reach statistical significance.

How do jury size, majority required and the number of verdicts available interact with each other?

Each unique feature of the Scottish jury system was independently and significantly related to the likelihood of individual jurors favouring a particular verdict in this research, in the ways described above. However, these features are also likely to interact in particular ways. Analysis of these interactions indicates that:

• The size of the majority required is the feature that has the biggest impact on the likelihood of individual jurors changing their view on which verdict should be returned. Jurors were more likely to change their view in juries asked to reach a unanimous verdict.
• The combination of features that produced the most jurors in favour of conviction after deliberating was 15-person, simple majority, two-verdict juries. In contrast, the combination which produced the lowest number of jurors favouring conviction was 12-person, unanimous, three-verdict verdict juries. Neither combination currently exists in practice, but the first combination (in which jurors were most likely to prefer a guilty verdict) is how the Scottish system would look if the not proven verdict were to be abolished without any other reforms taking place at the same time.

How do jurors understand the not proven verdict?

There was evidence of some inconsistency in jurors' understanding of what the not proven verdict means, along with some confusion over the consequences of not proven for the accused.

It should be stressed, however, that while there was some uncertainty over the meaning of the not proven verdict, jurors relatively rarely expressed beliefs about the verdict that were definitively incorrect. This is in part because the not proven verdict does not have a specific definition beyond it being one of two verdicts of acquittal. This leaves room for a number of different understandings of its meaning and purpose, which are explored below.

Across the 32 mock juries that had not proven as an option, the meaning and consequences of the not proven verdict were rarely discussed at any length during deliberations, even in juries where that verdict was returned. Where the not proven verdict was discussed, however, there was evidence of jurors holding inconsistent understandings of what the verdict meant, along with some confusion over its effect. In particular, jurors expressed uncertainty as to how it differed (if at all) from a not guilty verdict.

Although not proven and not guilty have the same effect in law, jurors tended to give different reasons for choosing them. Those who favoured the not proven verdict tended to base this on a belief that the evidence did not prove guilt beyond reasonable doubt, or on the difficulty of choosing between two competing accounts. Jurors choosing the not guilty verdict (where not proven was also an option), on the other hand, tended to attribute this to a belief that the accused was innocent or to some aspect of the complainer's or witness' evidence that suggested that they were not giving a truthful account.

The idea that the not proven verdict should be used when jurors think that the accused is probably guilty but that this has not been proven to the necessary standard arose frequently, albeit briefly, in deliberations. It was also the issue on which there was the clearest agreement in questionnaire responses. Jurors also expressed the view that there would be a lingering stigma attached to receiving a verdict of not proven.

Other implications: supporting juror understanding

As discussed above, there is no evidence from this study that changing one or more of the unique features of the Scottish jury system would have an impact on the number of evidential issues or on the extent or accuracy of legal issues discussed during deliberations.

However, the findings do raise important questions about what can be done to support jurors' understanding of legal issues, including their understanding of the meaning and effects of the not proven verdict. Several potential misunderstandings on the part of individual jurors arose relatively frequently across the mock juries (e.g. a belief that the accused should prove his innocence, a belief that the accused can be retried following a not proven verdict but not a not guilty verdict, and misunderstanding of the fact that self-defence is a legitimate defence to an assault charge, even when the fact the accused inflicted the injury is not in dispute). This suggests a need to consider whether additional guidance (such as written routes to verdict or written reminders of key legal principles) would be helpful to aid jurors' discussions. Another strand of this research involved an extensive evidence review of ways in which juror communication methods might be improved: see J Chalmers and F Leverick, Methods of Conveying Information to Jurors: An Evidence Review (2018).