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If you want to serve on a criminal jury, the most important rule is this: Say as little as possible, with your words, your body language, and your appearance.

But why would you want to sit on a jury in the first place? Because in a criminal trial, if you can read and reason and resist being swayed by emotion, you will make a better juror than most of your fellow members of the community. A jury is the entity that acts as the voice of the community, and serving as a juror allows you to contribute to that voice.

You may also believe that the law under which the defendant is being prosecuted is an illegitimate use of state power. In that case, acting as a juror gives you the opportunity to exercise the power of jury nullification—finding the defendant "not guilty" regardless of whether the state has proven the accusation beyond a reasonable doubt.

This power to nullify an unjust law is as old as the institution of the jury; it's a practice rooted in the principle that a juror can and should reach whatever verdict her conscience leads her to, and that there is nothing the government, or anyone else, can do to stop her beforehand or punish her afterward. Of course, the state prefers to maintain tight control over trials. In most jurisdictions, defense lawyers are prohibited from telling juries about nullification, and judges and prosecutors will, if pressed, lie and tell jurors they may not vote to nullify. But that is all the state can do to try to stop it. Knowing the truth will keep you from being deceived.

In 23 years of criminal defense practice, I've tried more than 40 cases before juries that I've picked, plus assisted and watched many more lawyers' jury selections. I've made a study of the psychology and social dynamics of the process and taught the science and practice of it to countless lawyers across the country. I've learned that getting onto a jury to nullify illegitimate laws is easier when you understand the game that judges and attorneys are playing.

You are an intelligent, opinionated person who wants to share with your fellow citizens the fact that they have the power to follow their consciences in arriving at a verdict. This is admirable. But if you succumb to the temptation to do so during jury selection, your chances of being chosen drop to nil.

We call the process of turning a group of community members into a jury of six or 12 "jury selection," but it is, by necessity, actually jury deselection. Each party can eliminate from the jury pool any person who has a bias for or against the defendant or a bias against any of the laws that are applicable to the case (this is a "challenge for cause"). Then each side can eliminate from the jury pool a fixed number of people for any reason at all, as long as that reason is not some form of proscribed discrimination (this is a "peremptory challenge"). The jury is the first dozen people (or half-dozen, in a misdemeanor case) remaining after both sides have exercised their challenges.

Lawyers find bias, and other reasons to strike jurors, in the things candidates say, the way they act, and how they look. As a practical matter, the first six or 12 people left after the lawyers have used all of their strikes are those who have kept their mouths shut and who appear ordinary.

Bias against "the law applicable to the case" is grounds for a challenge for cause, and while you and I know that jury nullification falls within the bounds of the law, the system in practice does not recognize that principle. Judges will bar defense lawyers from even mentioning jury nullification, and judges and prosecutors will lie to jurors about that power (or right, if you prefer, since the people's rights are, of course, powers in relation to the state). A juror who expresses any understanding of her power to nullify bad laws will certainly be challenged by the prosecutor for cause and excused by the judge. Precedent is very clear that a willingness to nullify the law is a bias against that law, which is grounds for a juror to be stricken for cause.

The first challenge for someone who wants to be able to exercise his own sense of right and wrong in the jury room is—to be blunt—not to let the state know that he plans to do so.

Potential jurors are questioned under oath. As a philosophical matter, a person may feel that where the court and the state are lying to jurors about their power to nullify, jurors are justified in lying back. Perhaps you feel the power to nullify a law contains the power to nullify the oath to tell the truth, if that is the only way to exercise your right. But for our current purposes, let's assume that you are unwilling to commit perjury for the sake of nullification—that you believe lying under oath is a greater evil than being excluded from a jury because you know about your right to nullify bad laws. In that case, if the prosecutor or the judge asks you explicitly about your power to nullify—"Ms. Jones, do you believe that a juror has a right to follow her conscience rather than the law?"—you will answer truthfully, and your truthful answer will likely get you excused from the jury without further questioning.

Lawyers in jury selection have a lot on their minds, however. They are also often afraid of "poisoning the well" by eliciting ideas they don't want the other potential jurors to hear (including ideas about nullification). Now, this fear is irrational, since a juror who holds such a view but keeps it to himself will quietly carry the "poison" into the jury room. But even lawyers behave irrationally, and the chance you will be confronted with such a clear and direct question is slim.

Slightly more likely is a query such as, "By show of hands, who here believes in jury nullification?"

If you parse the question and the answer is inescapably "yes," then not raising your hand is concealing facts from the court while under oath to tell the truth. If you do raise your hand, there will be additional questions, which may lead the judge to conclude that you have a bias against the law and should be stricken from the jury. At the very least, your speaking up will give the prosecution a reason to exercise a peremptory challenge against you.

Better, from your perspective, that prosecutors should use a peremptory challenge than a challenge for cause, since they have an unlimited number of the latter but very few of the former. Still, the objective of this exercise is to get on the jury. Fortunately, unless the defense lawyer has telegraphed to the prosecutor an intention to rely on nullification (a bad idea if she actually plans to do so, though an excellent diversion if she does not), you probably won't get this question, either.

What is likely—especially with a case involving a relatively unpopular law, such as a law criminalizing possession of marijuana—is that the prosecutor or judge will ask a question along the lines of, "How many of you think that possession of marijuana should not be against the law?"

Your job as a potential juror is not to make the lawyers' jobs easy for them. If lawyers ask bad questions, you are not obligated to guess at what they actually meant, nor are you obligated to respond to the phrasing they should have used. ("The law in this state is that it is a crime for an adult to possess marijuana, anywhere, at any time. How many of you think that should be the law?") In this instance, you may think it fair to say that "not against the law" means never, under any circumstances, against the law. If you can imagine a situation in which it might be legitimately outlawed (in a school zone by a kindergartner?), you can honestly refrain from volunteering your view.

Assuming you are forced during jury selection to reveal your familiarity with the practice of nullification, you will not be serving on the jury. If, after learning that you know about it, the prosecutor is unwise enough to allow you to say more, you might as well take the opportunity to educate your fellow jurors about the doctrine. At least then you'll have accomplished something.

Nullifying illegitimate laws is easier when you understand the game that judges and attorneys are playing.

A juror who admits he is not able to follow the law is challengeable for cause. That's a freebie for the state. But of course you are able to follow the law; you just don't agree with the judge's and the prosecutor's assessment of the state of it. You don't have to share that last bit unless hard pressed; "I can follow the law" is often sufficient to keep a prosecutor from successfully challenging you for cause. If you can promise to "set aside your beliefs about jury selection and follow the law," so much the better. And what does "set aside your beliefs" mean? Who knows. They are formalistic magic words.

If your beliefs about drug laws are closely examined, you may honestly need to disclose that you would define "proof beyond a reasonable doubt" more strictly in a drug case than in some other cases. That likely won't lead to a challenge for cause: Reasonable doubt is a matter of personal judgment, each juror gets to decide what it means to him or her, and the prosecution isn't allowed to pick only people who agree with its definition of the term.

That pretty well covers the words you say: as few as possible, preferably none, while adopting the interpretation of each question that allows you the most freedom to keep your mouth shut. But you also need to consider the things you say without words. Most of us give away a great deal of information with our body language. Canny lawyers in jury selection are watching your reactions—or have assistants doing so—and take that into account when making their peremptory challenges.

They also look at your clothing and accoutrements. So if you want to be on a jury, give them nothing to notice. Have a poker face, dress conservatively, and don't carry incendiary reading material, such as the latest issue of Reason magazine.

Once you get on the jury, you will want to make the most of it. It may be that the defendant is accused of a crime that is malum in se (i.e., inherently wrong); that the police acquired the evidence without violating the defendant's constitutional rights; and that the evidence proves the government's case beyond a reasonable doubt. If so, you will follow the law and vote to convict. But if those conditions are not all true, and if conscience demands that you not convict the defendant, you can try to nullify. You may be able to get the rest of the jury to go along with you and hand down an acquittal.

By the time the presentation of evidence begins, all of the jurors have an opinion on culpability. As soon as they go in the jury room, they take a first vote to see where everyone is. Lawyers like to pretend that jurors then calmly and rationally deliberate, but the truth is that the majority pressures, cajoles, and browbeats the minority to switch sides. The evidence matters only insofar as any juror can use it to shore up her own position or give another juror an excuse to change his.

Each person's vote is a personal moral judgment, and nobody is entitled to pressure another person to go against his belief. But most people are not able to withstand the sort of social pressure that is put on them in the jury room, and so the side with fewer jurors in that first vote is likely to lose this battle. The greater the gap, the more likely are people in the minority to defect.

Criminal verdicts have to be unanimous, so if the jury announces that it cannot come to a decision, the court will take a few steps. First it sends them back for more deliberating. Next it gives them an "Allen charge" or "dynamite charge"—a set of instructions from the bench specifically intended to push the jurors to break the deadlock. Only when the court is convinced that the jury is hopelessly hung will the court accept that outcome and declare a mistrial.

The government will then have to decide whether to retry the defendant. A mistrial is not an acquittal, but it's better than a conviction.

Jurors are not always informed about what happens in cases of disagreement in the jury room. You understand that there is light at the end of the tunnel even if the jury hangs, but many of your peers don't. This knowledge is power. If you are the lone nullifier, you have little chance of winning the other 11 (or five) people over to your point of view, except for this: They want to go home; they may not know whether that will happen if there is no agreement; and they are probably not as heavily invested in convicting the defendant as you are in preventing an unjust result. Those three factors give you a chance of participating in an acquittal instead of just a hung jury.

You are unlikely to get there by launching into a disquisition on the history of jury nullification in Anglo-American jurisprudence. Jurors swear to render "a true verdict according to the law and the evidence." You and I know that someone who nullifies a bad statute in order to acquit is not violating this oath—you have pledged to rule according to "the law," which includes the power to nullify. But because judges and prosecutors deny that, you may be making needless trouble for yourself by justifying your verdict in nullification terms. Your fellow jurors could complain to the judge, who will tell them nullification is not the law, thereby setting them more firmly against you. Depending on her level of legal ignorance, your judge also might take other action against you, such as removing you from the jury or holding you in contempt.

Because your verdict is your own personal moral judgment, you have no obligation to explain or justify it to anyone. But if you want to see the defendant acquitted, you need to give your fellow jurors some face-saving justification for moving from "guilty" to "not guilty."

Reasonable doubt is a good place to start, because it is a nebulous standard: A smart person can always find a doubt, and she can usually, if she wants to, find some rationalization for it—a reason that it is reasonable—as well. During the trial, the defense lawyer should have given you ammunition for convincing those of your peers who are inclined to convict that there is reasonable doubt in the case.

If you show a steadfast dedication to your position and you can give the other jurors some plausible reason to doubt the prosecution's case, you might just be able to turn your one vote into two, two into four, and so forth—it gets easier as you have more people on your side—until finally you've turned a hung jury into an acquittal.

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