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|The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (June 2013)|
|Part of the common law series|
|Element (criminal law)|
|Scope of criminal liability|
|Seriousness of offense|
|Offence against the person|
|Crimes against property|
|Crimes against justice|
|Defences to liability|
|Other common law areas|
An affirmative defense to a civil lawsuit or criminal charge is a fact or set of facts other than those alleged by the plaintiff or prosecutor which, if proven by the defendant, defeats or mitigates the legal consequences of the defendant's otherwise unlawful conduct. In civil lawsuits, affirmative defenses include the statute of limitations, the statute of frauds, and waiver. In criminal prosecutions, examples of affirmative defenses are self defense, insanity, and the statute of limitations.
In an affirmative defense, the defendant may concede that he committed the alleged acts, but he proves other facts which, under the law, either justify or excuse his otherwise wrongful actions, or otherwise overcome the plaintiff's claim. In criminal law, an affirmative defense is sometimes called a justification or excuse defense. Consequently, affirmative defenses limit or excuse a defendant's criminal culpability or civil liability.
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 another's use of force was unlawful and that the defendant's conduct was necessary to protect himself.
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. The issue of timely assertion is often the subject of contentious litigation.
"Mistake of fact" is not an affirmative defense: it does not require proof but it does introduce doubt. In mistake-of-fact defenses, the defendant asserts that his mistaken belief prevents the establishment, beyond a reasonable doubt, of the required mens rea. It can be used with other defenses such as self-defense. Self-defense would still be available even if the defendant mistakenly believes that he was in imminent danger of harmful or offensive bodily contact.
Among the most controversial affirmative defenses is the insanity defense, whereby a criminal defendant seeks to be excused from criminal liability on the ground that a mental illness, at the time of the alleged crime, prevented him from understanding the wrongful nature of his actions.
In an affirmative defense the burden of proof is generally on the defendant to prove his allegations either by the preponderance of the evidence or clear and convincing evidence, as opposed to ordinary defenses (claim of right, alibi, infancy, necessity, and (in some jurisdictions, e.g., New York) self-defense (which is an affirmative defense at common law)), for which the prosecutor has the burden of disproving beyond a reasonable doubt.
Because an affirmative defense requires an assertion of facts beyond those claimed by the plaintiff, generally the party who offers an affirmative defense bears the burden of proof. The standard of proof is typically lower than beyond a reasonable doubt. It can either be proved by clear and convincing evidence or by a preponderance of the evidence.
Rule 8 of the Federal Rules of Civil Procedure governs the assertion of affirmative defenses in civil cases that are 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, estoppel, failure of consideration, fraud, illegality, injury by fellow servant, laches, license, payment, release, res judicata, statute of frauds, statute of limitations, waiver, and any other matter constituting an avoidance or affirmative defense."
Rule 11 of the Federal Rules of Civil Procedure requires that affirmative defenses be based on "knowledge, information, and belief, formed after an inquiry reasonable under the circumstances," and cannot consist of a laundry list of all known affirmative defenses.
An affirmative defense is different from a "negating defense". A negating defense is one which tends to disprove an element of the plaintiff's or prosecutor's case. An example might be a mistake of fact claim in a prosecution for intentional drug possession, where the defendant asserts that he or she mistakenly believed that the object possessed was an innocent substance like oregano. Because this defense simply shows that an element of the offense (knowledge of the nature of the substance) is not present, the defendant does not have any burden of persuasion with regard to a negating defense. At most the defendant has the burden of producing sufficient evidence to raise the issue.