Did you know that in the states of Oregon and Louisiana a defendant is convicted if 10 out of 12 jurors vote guilty? In Federal trials and in all other states unanimity is required.  A recent case was appealed to the Supreme Court challenging Oregon’s non-unanimous juries on 14th Ammendment grounds. On Monday the Court declined to hear the case.  (Here is Eugene Volokh, who brought the petition.)

This opinion piece in the Washington Examiner argues that the unanimity requirement is essential for preserving “liberties.”  I assume that what the author means is protection against convicting the innocent.  Because on its face it would seem that such a mistake is less likely when unanimous agreement of all 12 jurors is required.

Of course we should care not just about the error of convicting the innocent, but also acquitting the guilty.  But even if your concept of liberty puts maximum weight on the protection of the innocent, it is naive to suppose that this is achieved by unanimous juries.

Suppose you are on a jury in Oregon and the foreman has joined with 8 others who have decided to convict.  Looking for the 10th vote, he turns to you. Compare your incentives to convict in this situation to the analogous situation where, in Illinois, 11 others are looking your way.  With only 9 others prepared to convict there is not only less peer pressure on you, but other things equal the evidence is less persuasive.  It has only convinced 9 others.

All jurors see the same evidence but each views it from his or her own perspective. When a jury votes the jurors are signaling to one another how they interpret the evidence.  The more other jurors voting to convict, the stronger is your inference that the evidence shows the defendant is guilty. When you are the pivotal 10th juror you know only that 9 others have concluded that the defendant is guilty.

The lower threshold for conviction in fact makes you less likely to vote to convict.

This strategic effect of course has to be weighed against the mechanical effect of lowering the threshold and the net effect could go either way.  However, there is one unambiguous sense in which unanimous jury standards are in fact the worst possible.

In a famous paper, Feddersen and Pesendorfer showed that jury voting is informationally efficient in the following sense:  given enough jurors with enough independent information the strategic effect outlined above is dampened.  And the defendant is convicted if and only if he is guilty.

Now in a sense this is purely of theoretical interest.  Juries of 12 are not “arbitrarily large” and even if they perfectly pool their information they will make mistakes. But the point of this result is that it says that jury voting in principle works. Feddersen and Pesendorfer showed that this is true regardless of the threshold fraction of votes required for conviction, but with one single exception.  Under unanimity rule the strategic effect is not dampened.  Indeed with more and more jurors, knowing that you are the last holdout is stronger and stronger evidence that you should convict.  Thus there is always a probability of convicting the innocent even with very large juries.

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