An affirmative defense is a defense used in litigation between private parties in common law jurisdictions, and is, more commonly, a defense raised in criminal law by the defendant . Affirmative defenses work to limit or excuse a defendant's criminal culpability or civil liability, even if the plaintiff's claim is proven, based on facts outside those claimed by the plaintiff.

The best-known affirmative defense in popular culture is the insanity defense, whereby a criminal charged even with murder may be institutionalized in an asylum rather than imprisoned if it can be shown that they were legally insane at the time of the crime.

An affirmative defense must be timely pleaded by the defendant in order for the court to consider it, or else it is considered waived by the defendant's failure to assert it. What constitutes timely assertion is often itself the subject of contentious litigation.

Because affirmative defenses require the assertion of facts beyond those claimed by the plaintiff, generally the party making an affirmative defense has the burden of proof on that defense. The burden 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.

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.

Examples

• civil law
  • merger doctrine

• • contributory negligence (when the plaintiff's actions contributed to his own injury)
  • assumption of risk (when the plaintiff knowingly entered into a dangerous situation)
  • duress
  • statute of frauds
  • statute of limitations
  • contract-specification defense
  • contractual provision (when the defendant's liability for causing the plaintiff's injuries had been waived in the contract)
• criminal law
  • insanity defense
  • self-defense
  • necessity

Civil law | Criminal defenses