Abstract
We raise the question whether there is a rigorous argument favoring one jury system over another. We provide a Bayesian model of deliberating juries that allows for computer simulation for the purpose of studying the effect of jury size and required majority on the quality of jury decision making. We introduce the idea of jury value (J-value), a kind of epistemic value which takes into account the unique characteristics and asymmetries involved in jury voting. Our computer simulations indicate that requiring more than a > 50 % majority should be avoided. Moreover, while it is in principle always better to have a larger jury, given a > 50 % required majority, the value of having more than 12–15 jurors is likely to be negligible. Finally, we provide a formula for calculating the optimal jury size given the cost, economic or otherwise, of adding another juror.
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- 1.
Williams v. Florida, reprinted as pp. 3–70 in Jacobstein and Mersky (1998).
- 2.
- 3.
- 4.
In the Scottish legal system, a jury can also give the verdict “not proven”. As some commentators (e.g. Luckhurst 2005) have noted, including this verdict alongside the not guilty verdict has no legal consequence: in both cases the accused goes free and cannot be tried again for the same offence. Instead it is common to argue that the value of the not proven verdict is not legal but social: it allows the jury, for better or worse, to acquit the defendant while leaving a stain on his or her character. Alternatively, it can be used to “expose a poor investigation and highlight the failings of an incompetent prosecutor” (Luckhurst 2005). Since the “not proven” verdict has no legal consequence we have decided not to take it into account in this study of legal decision making.
- 5.
In some jury systems, such as the American, the condition specifying when the deliberation has come to an end does not refer to time but to some other feature of the situation, such as the jurors having reached a unanimous verdict. We have decided to leave the study of such jury systems for another paper. Having said this, the simulation results we present below count indirectly against the American system, and it seems unlikely, in light of these results, that the latter should be a serious competitor to e.g. the Scottish jury system as regards the quality of collective decision making.
- 6.
For more on the PAT tradition see Isenberg (1986).
- 7.
This principle has reappeared in many guises both before and after Blackstone, with a varying number of guilty acquittals held to be better than one innocent conviction. Cf. Volokh (1997).
- 8.
- 9.
It is, of course, always possible to scale the J-values so that they have the same maximum and minimum, thereby achieving an illusion of compatibility. But without an independent argument for why these maxima and minima should be the same such an approach seems woefully ad hoc.
- 10.
We owe the observation that there might be a free-riding problem in larger juries to Andrzej Wiśniewski.
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Acknowledgements
This paper was written by Angere and Olsson, except the second part of Sect. 6, which was written by Genot.
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Angere, S., Olsson, E.J., Genot, E.J. (2016). Inquiry and Deliberation in Judicial Systems: The Problem of Jury Size. In: Başkent, C. (eds) Perspectives on Interrogative Models of Inquiry. Logic, Argumentation & Reasoning, vol 8. Springer, Cham. https://doi.org/10.1007/978-3-319-20762-9_3
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