Jury nullification is the legal power of a jury to declare a defendant "not guilty" even though the law would require a guilty verdict. In so doing, the jury "nullifies" the effect of the law in a particular case.
The general power of juries to decide on verdicts was stated in the English Magna Carta of 1215, which put into words existing practices:
No free man shall be captured, and or imprisoned, or disseised of his freehold, and or of his liberties, or of his free customs, or be outlawed, or exiled, or in any way destroyed, nor will we proceed against him by force or proceed against him by arms, but by the lawful judgment of his peers, or by the law of the land
For a trivial offence, a free man shall be fined only in proportion to the degree of his offence, and for a serious offence correspondingly, but not so heavily as to deprive him of his livelihood. In the same way, a merchant shall be spared his merchandise, and a husbandman the implements of his husbandry, if they fall upon the mercy of a royal court. None of these fines shall be imposed except by the assessment on oath of reputable men of the neighbourhood.
Even though the Magna Carta guaranteed trial by jury, juries were expected to follow the instructions of the judge or of the crown. Judges had the right to order a retrial of a case if the judge believe that the first jury reached the wrong verdict.
A famous jury nullification case was the trial of William Penn in England. Penn had joined the Quakers in London, a religion disliked by the King. In 1670, Penn held a worship service and was arrested allegedly for disturbing the King's peace At trial, the jurors heard testimony and the judge, as is still the custom, delivered jury instructions prior to its deliberations. The jury instructions included an order to find the Penn guilty.
But the jury refused to find Penn guilty. The judge angrily sent them back to continue deliberations. The jury returned again with its same verdict. The judge demanded "a verdict that the court will accept, and you shall be locked up without meat, drink, fire, and tobacco .... We will have a verdict by the help of God or you will starve for it." The jury went out three more times, and returned with the same verdict each time. Then it refused to deliberate any more and the judge fined and imprisoned them. Penn was also fined and imprisoned on a scurrilous new charge invented at trial (for donning a hat in the courtroom).
On appeal, the jurors won their independence and were released from jail, though that did not help Penn.
This case established the principle that a juror could not be prosecuted for his vote on the jury, whereby a jury’s decision is final and dispositive even if it rejects the law. Jury nullification has occurred in America, as in acquitting John Peter Zenger and acquitting defendants accused of violating the Alien and Sedition Act and the Fugitive Slave Act.
Today, court rules and the ethics rules governing attorney conduct prohibit lawyers and judges from suggesting to juries that is jury nullification is an available option. Each juror takes an oath to apply the law as instructed by the judge, and jurors are asked questions during jury selection to determine whether they are prepared to carry out that duty. For example, in death penalty cases, prospective jurors are asked if their views on the death penalty would affect their ability to perform their duty. So, for jury nullification to occur in the United States, a juror would not only break his oath but also be prepared to lie about his views and intent during jury selection. An additional safeguard is that if the jury was not uninanimous, a "hung jury" would result in a mistrial and a subsequent trial by a different jury.
U.S. v. Thomas, 116 F.3d 606 (2d Cir. 1997).