Affirmative defense

An affirmative defense is a category of defense used in litigation between private parties in common law jurisdictions, or, more familiarly, a type of defense raised in criminal law by the defendant. Many affirmative defenses can be classified as justification defenses, or excuse defenses. Affirmative defenses operate to limit, excuse or avoid a defendant's criminal culpability or civil liability, even if the facts of the plaintiff's claim are admitted or proven. In fact, the defendant usually must affirmatively come forward with some evidence that the defense exists; hence, "affirmative" defenses.

A clear illustration of an affirmative defense is self-defense. In its simplest form, a criminal defendant may be exonerated if he can demonstrate that he had an honest and reasonable belief that his conduct was necessary to protect himself from another's use of unlawful force.

"Mistake of fact" is another affirmative defense, usually used in combination with another, in which the defendant asserts that he reasonably believed the culpable act was necessary based on his observation, even if, when given complete knowledge of the situation, the act may have been unwarranted. Self-defense is still available even if the defendant's belief that he was in imminent danger of harmful or offensive bodily contact was mistaken.

Among the most controversial affirmative defenses is the insanity defense, whereby a criminal defendant, shown to be insane at the time of his crime, seeks commitment to a mental institution in lieu of imprisonment.

Most affirmative defenses must be pled in a timely manner by a defendant in order for the court to consider them, or else they are considered waived by the defendant's failure to assert them. The classic unwaivable affirmative defense is lack of subject-matter jurisdiction. What constitutes timely assertion is often itself the subject of contentious litigation.

Because an affirmative defense requires an assertion of facts beyond those claimed by the plaintiff, generally the party making an affirmative defense bears the burden of proof. The standard of proof is typically lower than beyond a reasonable doubt. It can either be proof by clear and convincing evidence or a preponderance of the evidence. In some cases or jurisdictions, however, the defense must only be asserted, and the prosecution has the burden of proving beyond a reasonable doubt that the defense is not applicable.

Rule 8 of the Federal Rules of Civil Procedure governs the assertion of affirmative defenses in civil cases filed in the United States district courts. Rule 8(c) specifically enumerates the following defenses: "accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, discharge in bankruptcy, duress, estoppel, failure of consideration, fraud, illegality, and any other matter constituting an avoidance or affirmative defense."

An affirmative defense can be different from a negating defense. A negating defense is one which tends to negate an essential element of the state's case. An example might be a mistake of fact claim in a prosecution for intentional drug possession, where the defendant asserted that he or she mistakenly believed that the object possessed was an innocent substance like oregano. Because this defense simply shows that an essential element of the offense is not present, the defendant does not have any burden of persuasion with regards to a negating defense. At most the defendant has the burden of producing sufficient evidence to raise the issue.