The effect of verdict system on juror decisions: a quantitative meta-analysis.

We study the effect of the Scottish three-verdict system (guilty, not guilty, not proven) and the Anglo-American two-verdict system (guilty, not guilty) on juror decisions by combining data sets from 10 mock trials reported in suitable studies. A logistic regression with random effects uses the exact number of convictions and acquittals in 10 mock trials from a total of 1778 jurors to reliably estimate the effect of verdict system. We found a statistically significant verdict effect suggesting that the odds for a conviction by a juror are about 0.6 times or 40% lower under the three-verdict system than under a conventional two-verdict system. Possible explanations and implications of this verdict effect are discussed. This finding helps to better understand juror decision making in the context of the current reform of the Scottish three-verdict system into a two-verdict system.