Jury nullification occurs in a trial when a jury acquits a defendant, even though the members of the jury may believe that the defendant did the illegal act, yet they don't believe he should be punished for it. This may occur when members of the jury disagree with the law the defendant has been charged with breaking, or believe that the law should not be applied in that particular case. A jury can similarly unjustly and illegally convict a defendant on the ground of disagreement with an existing law, even if no law is broken (although in jurisdictions with double jeopardy rules, a conviction can be overturned on appeal, but an acquittal cannot).A jury verdict that is contrary to the letter of the law pertains only to the particular case before it. If a pattern of acquittals develops, however, in response to repeated attempts to prosecute a statutory offence, this can have the de facto effect of invalidating the statute. A pattern of jury nullification may indicate public opposition to an unwanted legislative enactment.In the past, it was feared that a single judge or panel of government officials may be unduly influenced to follow established legal practice, even when that practice had drifted from its origins. In most modern Western legal systems, however, judges often instruct juries to serve only as ""finders of facts"", whose role it is to determine the veracity of the evidence presented, the weight accorded to the evidence, to apply that evidence to the law, and to reach a verdict; but not to question the law or decide what it says. Similarly, juries are routinely cautioned by courts and some attorneys not to allow sympathy for a party or other affected persons to compromise the fair and dispassionate evaluation of evidence during the guilt phase of a trial. These instructions are criticized by advocates of jury nullification. Some commonly cited historical examples of jury nullification involve jurors refusing to convict persons accused of violating the Fugitive Slave Act by assisting runaway slaves or being fugitive slaves themselves, or for breaking the refusal of American colonial juries to convict a defendant under English law.Juries have also refused to convict due to the perceived injustice of a law in general, or the perceived injustice of the way the law is applied in particular cases. There have also been cases where the juries have refused to convict due to their own prejudices such as the race of one of the parties in the case.