Jury nullification (US) or a perverse verdict (UK) generally occurs when members of a criminal trial jury believe that a defendant is guilty, but choose to acquit him anyway because the jurors also believe that the law itself is unjust, that the prosecutor has misapplied the law in the defendant's case, or that the potential punishment for breaking the law is too harsh. Some juries have also refused to convict due to their own prejudices in favour of the defendant.
Nullification is not an official part of criminal procedure, but is the logical consequence of two rules governing the systems in which it exists:
Jurors cannot be punished for reaching a "wrong" decision (such as acquitting a defendant despite their guilt being proven beyond a reasonable doubt).
A defendant who is acquitted cannot be tried again for the same alleged crime in front of another jury.
A jury verdict that is contrary to the letter of the law pertains only to the particular case before it. However, if a pattern of acquittals develops in response to repeated attempts to prosecute a particular offence, this can have the de facto effect of invalidating the law. Such a pattern may indicate public opposition to an unwanted legislative enactment. Likewise, a jury can convict a defendant even if no law was actually broken, although such a conviction may be overturned on appeal. Nullification can also occur in civil trials.