First, prosecutors know the facts needed to make decisions in the name of justice while juries generally don’t. Prosecutors are supposed to make a decision to prosecute after learning things like the suspect’s criminal record, the full scope of his conduct (including the inadmissible parts), how much a prosecution might deter future crimes, and what the punishment might be if the suspect is convicted. Prosecutors can get the facts and make a call.
We might disagree with a prosecutor’s decision, of course. But the prosecutor at least has access to the information needed to make the decision.
Jurors usually don’t have that information. Jurors are not told what they would need to know to decide what is just. We keep such information away from jurors to help ensure a fair trial and preserve other values in the criminal justice system. The jurors normally don’t know about the defendant’s criminal record and past bad acts, as we don’t want the jury to just assume that someone who has done bad things before is probably guilty this time, too. Jurors aren’t told of the inadmissible evidence, such as evidence excluded under the Fourth, Fifth, and Sixth Amendment, to encourage compliance with those provisions of the Constitution. And we don’t explain to jurors why a particular prosecution is thought to further the purposes of punishment because, among other reasons, doing so would take a lot of time and distract jurors from the question of guilt or innocence.
In that system, encouraging jury nullification is a recipe for arbitrariness instead of informed judgment.
Second, jury discretion is less democratically accountable than prosecutorial discretion. Criminal prosecutions are democratically accountable in two ways. First, before the crime occurs, the elected legislature must enact a law saying that, in general, the conduct should be punished. Second, after the crime occurs, elected executive officials and their employees must make a judgment that the specific conduct by the specific individual merits prosecution. Because prosecutors are repeat players who work for elected politicians, prosecutorial decisions in the aggregate are ultimately subject to review by a majority of the voters. If the voters don’t like how a prosecutor’s office has exercised discretion, the voters normally can vote to throw out the head of the office. Both the general judgment ex ante and the specific judgment ex post have to match for a prosecution to be brought.
It’s a different picture with juries. You might think of juries as a representative of “the People” and therefore assume they are democratically accountable. But note that in criminal cases, the law normally requires juries to be unanimous in order to render a guilty verdict. It takes only a single juror to block a conviction. The evidence can be overwhelming, and eleven of the jurors can believe fervently that a particular case is the most compelling prosecution ever brought. But a single juror, accountable to no one, can put the kibosh on the case based on his own vision of justice that may have no connection to anyone else’s. We don’t normally think of placing all the power in one unelected person who answers to no one as a democratically accountable approach.
I recognize the intuitive appeal of jury nullification. If you don’t like a particular kind of case that keeps being brought, jury nullification might look like a way to bring about a better world. If you’re the juror, your nullification can singlehandedly undo the decisions of the legislators and executive officials (and the sheep who voted them into office) who are so obviously wrong about the public interest. The more confident you are in your abilities to understand what others don’t, the better jury nullification sounds. But consider that people with your wisdom and judgment can’t be on every jury. When you consider all the juries, the effect of encouraging nullification is likely to make the system more arbitrary and less accountable rather than more wise.
Finally, some of the arguments for jury nullification sound like arguments for executive clemency in slight disguise. Executive clemency should be a critical part of any criminal justice system. Decisions to grant executive clemency are made by a democratically accountable office, however, after a review of all the facts. That’s not true with jury nullification.