IF you are ever on a jury in a marijuana case, I recommend that you vote “not guilty” — even if you think the defendant actually smoked pot, or sold it to another consenting adult. As a juror, you have this power under the Bill of Rights; if you exercise it, you become part of a proud tradition of American jurors who helped make our laws fairer.

The information I have just provided — about a constitutional doctrine called “jury nullification” — is absolutely true. But if federal prosecutors in New York get their way, telling the truth to potential jurors could result in a six-month prison sentence.

Earlier this year, prosecutors charged Julian P. Heicklen, a retired chemistry professor, with jury tampering because he stood outside the federal courthouse in Manhattan providing information about jury nullification to passers-by. Given that I have been recommending nullification for nonviolent drug cases since 1995 — in such forums as The Yale Law Journal, “60 Minutes” and YouTube — I guess I, too, have committed a crime.

The prosecutors who charged Mr. Heicklen said that “advocacy of jury nullification, directed as it is to jurors, would be both criminal and without constitutional protections no matter where it occurred.” The prosecutors in this case are wrong. The First Amendment exists to protect speech like this — honest information that the government prefers citizens not know.

Laws against jury tampering are intended to deter people from threatening or intimidating jurors. To contort these laws to justify punishing Mr. Heicklen, whose court-appointed counsel describe him as “a shabby old man distributing his silly leaflets from the sidewalk outside a courthouse,” is not only unconstitutional but unpatriotic. Jury nullification is not new; its proponents have included John Hancock and John Adams.

The doctrine is premised on the idea that ordinary citizens, not government officials, should have the final say as to whether a person should be punished. As Adams put it, it is each juror’s “duty” to vote based on his or her “own best understanding, judgment and conscience, though in direct opposition to the direction of the court.”

In 1895, the Supreme Court ruled that jurors had no right, during trials, to be told about nullification. The court did not say that jurors didn’t have the power, or that they couldn’t be told about it, but only that judges were not required to instruct them on it during a trial. Since then, it’s been up to scholars like me, and activists like Mr. Heicklen, to get the word out.

Nullification has been credited with helping to end alcohol prohibition and laws that criminalized gay sex. Last year, Montana prosecutors were forced to offer a defendant in a marijuana case a favorable plea bargain after so many potential jurors said they would nullify that the judge didn’t think he could find enough jurors to hear the case. (Prosecutors now say they will remember the actions of those jurors when they consider whether to charge other people with marijuana crimes.)

There have been unfortunate instances of nullification. Racist juries in the South, for example, refused to convict people who committed violent acts against civil-rights activists, and nullification has been used in cases involving the use of excessive force by the police. But nullification is like any other democratic power; some people may try to misuse it, but that does not mean it should be taken away from everyone else.

How one feels about jury nullification ultimately depends on how much confidence one has in the jury system. Based on my experience, I trust jurors a lot. I first became interested in nullification when I prosecuted low-level drug crimes in Washington in 1990. Jurors here, who were predominantly African-American, nullified regularly because they were concerned about racially selective enforcement of the law.

Across the country, crime has fallen, but incarceration rates remain at near record levels. Last year, the New York City police made 50,000 arrests just for marijuana possession. Because prosecutors have discretion over whether to charge a suspect, and for what offense, they have more power than judges over the outcome of a case. They tend to throw the book at defendants, to compel them to plead guilty in return for less harsh sentences. In some jurisdictions, like Washington, prosecutors have responded to jurors who are fed up with their draconian tactics by lobbying lawmakers to take away the right to a jury trial in drug cases. That is precisely the kind of power grab that the Constitution’s framers were so concerned about.

In October, the Supreme Court justice Antonin Scalia, asked at a Senate hearing about the role of juries in checking governmental power, seemed open to the notion that jurors “can ignore the law” if the law “is producing a terrible result.” He added: “I’m a big fan of the jury.” I’m a big fan, too. I would respectfully suggest that if the prosecutors in New York bring fair cases, they won’t have to worry about jury nullification. Dropping the case against Mr. Heicklen would let citizens know that they are as committed to justice, and to free speech, as they are to locking people up.