Jury nullification is when one or more members of a jurors find a defendant not guilty even though they believe the defendant broke the law. Lawyers spend a lot of time choosing who sits on a jury because they know many people have beliefs that override laws.
In 1990 a San Diego newspaper ran a jury nullification ad placed by an anti-abortion group aimed at potential jurors in a trial against three activists charged with trespassing on an abortion clinic. The ad said:
"The most important rule is, don't let the judge and prosecutor know that you know about this right. It is unjust and illegal for them to deny you this right. So, if you have to, it's perfectly all right for you to make a 'mental reservation.'
"Give them the same answer you would have given if you were hiding fugitive slaves in 1850 and the 'slave catchers' asked if you had runaways in your attic. Or if you were hiding Jews from the Nazis in Germany. The second rule is, educate the other jurors about jury nullification and, if possible, persuade them to vote 'not guilty.'"
According to the LA Times article, Deputy City Atty. Steve Miller said "the only time he knew of its having been used was in the South, at a time when Ku Klux Klan members were on trial for having murdered blacks, who, in Miller's words, 'were trying to exercise their rights, by registering to vote.'"
But now jury nullification is being mentioned again as a way to acquit abortion clinics and women who've had abortions in states with forced-birth laws. If the idea spreads widely, it could make prosecutors less likely to move ahead with some cases. From Dorf on Law:
Consider Texas's "trigger" law on abortions—a near total ban—set to automatically go into effect 30 days after Roe is overturned. What is the likelihood of drawing a juror who would consider nullification in a prosecution under this statute? In many, many possible cases, the likelihood seems high. Nearly 93% of abortions occur in the first 13 weeks of pregnancy. Thus, in 93% of cases—nearly all potential prosecutions—over 40% of potential jurors, on average, will view the prosecution as unjust, no matter the other facts. And an additional 22%—for a total of over 60%—might view it as such, depending on the circumstances. The odds of nullification rise commensurately as the prosecution becomes more extreme. In cases involving rape, serious fetal disability, or serious maternal health risk (though falling short of death or major disability), almost all jurors will view conviction as either certainly or potentially unjust.
Prosecutors will take note. This is critical. It is not the rate of actual nullifications that matters, but rather the equilibrium effects that the probability of nullification generates. Actual nullification may happen rarely precisely because prosecutors foresee its possibility and adjust their behavior accordingly. Given the above polling, how likely are prosecutors to ask a jury to convict a doctor for performing a 6th-week abortion of a fetus with a deadly chromosomal disorder? If prosecutors apprehend the likelihood of nullification, we think it unlikely. What about prosecuting a 14th week abortion to allow the mother treatment for a serious, but likely non-fatal, cancer discovered during the course of the pregnancy? Here again, if prosecutors understand the possibility of nullification, we think it would be unlikely. Prosecutions are more likely regarding abortions performed for family planning reasons. Though even here, early-term cases may be hard to win.
And contrary to the idea that it's against the law to tell potential jurors about nullification, the ACLU says "It's perfectly constitutional to talk about jury nullification."
Even Supreme Court Justice Atonin Scalia thought jury nullification was an appropriate way for citizens to fight back against unfair laws. From a 2011 NY Times opinion piece:
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."