In USA Today, Glenn Reynolds defends jury nullification: Nullifying juries more interested in justice than some prosecutors. Among other points:
As Clay S. Conrad notes in his Jury Nullification: The Evolution Of A Doctrine, to the framers of our Constitution, jury nullification was itself a feature, not a bug. Distrustful of the bureaucracy and even of the judiciary, framing-era Americans viewed a jury’s refusal to convict as an important protection for liberty. This remained the case until, Conrad notes, juries began refusing to enforce the Fugitive Slave Act of 1850, because they thought returning escaped slaves to their owners was unjust. In response, the system began trying to get around juries’ power not to convict. These efforts increased when juries were reluctant to convict labor leaders, or to enforce Prohibition. And though there were racist juries that refused to convict racist defendants in the civil rights era, Conrad notes that those juries were part of a system that also involved racist prosecutors, racist police, and racist judges.
I haven't read Conrad's book, but if his claims are correct I would think that jury nullification is constitutionally protected. That is, it is a core aspect of the right to trial by jury that the jury be able to acquit where it believes the law is unjust. As Renne Lettow Lerner points out in her interesting scholarship on civil juries and original meaning, it probably isn't (and couldn't be) the case that the jury clauses were intended to assure in perpetuity a jury trial that looked exactly like juries trials of 1789. On the other hand, though, surely at least core features of the jury trail must be protected, or the right would be meaningless — otherwise one could have a sham trial, with none of the actual protections that the clause was supposed to protect. The jury's ability to assess the justness of the law (assuming eighteenth century juries had that ability) seems like a core feature in this sense.
As a further aside, although I know little about the original conduct of jury trials, I do know that in the neutrality prosecutions of 1793 juries refused to convict people such as Gideon Henfield (an American who served on a French ship fighting the British even though the U.S. was officially neutral in the conflict between Britain and France). The Henfield verdict (and others like it) were interpreted at the time as the juries' rebuke to prosecutors, who had brought prosecutions for violations of neutrality even though no statute prohibited the defendants' actions. And, people at the time generally thought the juries had acted appropriately (although the Washington administration, which brought the prosecutions, did not).
(More responses on jury nullification from Ilya Somin and Orin Kerr at Volokh Conspiracy here and here).
Posted at 6:58 AM