Nullification is generally not a clearly-defined right protected by legal statute. Rather, it is an inherent aspect of the jury system under common law, sometimes justified as a safeguard of last-resort against wrongful imprisonment and government tyranny. Nullification actually derives from a pair of separate common law precedents: the prohibition on punishing jury members for their verdict, and a similar prohibition on retrying defendants after a not-guilty verdict has been handed down.
Until the late 17th century, the right of jurors to override the judge's instructions in returning not-guilty verdicts was a subject of contention in England. In 1670, William Penn was arrested for illegally preaching a Quaker sermon. Despite the fact that the judge demanded a guilty verdict, the jury in that case acquitted Penn and was imprisoned and fined as a result. The highest court in England released them and established a lasting precedent by ruling that jurors could not be punished for their decisions.
The use of the jury to act as a protection of last-resort was espoused by many of the Framers of the U.S. Constitution. As a result, the United States has a particularly strong legal tradition protecting the right of jury nullification. Though the right of a jury to nullify a verdict has been repeatedly affirmed by the U.S. Supreme Court, the 1895 decision of Sparf vs U.S held that a trial judge has no responsibility to inform the jury of that right. This decision, often cited, has led to a common practice in which juries are instructed that they must find guilt or innocence according to the law.
Advocacy groups such as the Fully Informed Jury Association work to inform potential jurors of their rights, and lobby for changes in the law requiring that judges properly inform jurors of their rights.Jury nullification in the United States