Affirmative Defense

The structure of criminal liability or guilt in Anglo-American law is straightforward. (The same structure of liability applies in general in civil suits, but this entry focuses on criminal law.) Crimes are defined by their criteria, which lawyers call the “elements.” Most crimes require some prohibited action and an accompanying mental state (the mens red). For example, one definition of murder is the intentional killing of a human being. The prohibited action is any type of killing conduct and the required mental state is intent, the purpose to kill. The State has enormous discretion concerning what behavior to criminalize and what the specific elements of crimes should be. The State (prosecution) must prove these elements beyond a reasonable doubt. Even if the prosecution is able to prove all the elements beyond a reasonable doubt, the defendant may nonetheless avoid criminal liability and be found not guilty (or less guilty) by establishing a defense. These defenses are termed affirmative defenses and, like the definitions of crimes, have definitional criteria. Affirmative defenses may be grouped into three categories: justifications, excuses, and policy defenses. The former two focus on the defendant's culpability or blameworthiness. The State creates the latter to serve goals other than adjudicating guilt. The State has enormous discretion concerning what affirmative defenses to establish, if any, and what their criteria should be. In the United States, it is in the State's discretion to allocate the burden of proof on affirmative defenses to either the prosecution or the defense.

Behavior that would otherwise be criminal is justified if it is right or at least permissible in the individual circumstances. For example, the intentional killing of another person is typically criminal homicide, but if someone kills in response to a wrongful and imminent threat of deadly harm, that person will be justified by the affirmative defense of self-defense. Other traditional justifications include the defense of another, the defense of property, law enforcement, and the general justification of “necessity” or “balance of evils,” which is often established to address cases in which the more specific justifications do not strictly apply. The defendant will be justified only if he or she actually believes that the justifying circumstances exist and that belief is reasonable. There is often substantial dispute about the criteria for a reasonable belief. A defendant found not guilty because his or her conduct was justified is freed outright from state control.

Criminal behavior is excused if the defendant was not criminally responsible at the time of the crime. For example, suppose someone intentionally kills because severe mental disorder produces a delusion that he or she is about to be killed. The individual is not justified because the belief is mistaken and unreasonable, but this person is sufficiently irrational to be considered nonresponsible, and the excuse of legal insanity applies. Other traditional excuses include infancy, which excuses from criminal responsibility juveniles below a certain age, and duress, which excuses an individual who is wrongfully threatened with death or serious bodily injury unless he or she commits a crime and a person of reasonable firmness would have yielded to the threat under the circumstances. A defendant who is excused may be subject to further state noncriminal control if the person remains dangerous. For example, a defendant who is found not guilty by reason of insanity may be civilly committed to a secure hospital if he or she remains dangerous and may be kept there until he or she is no longer mentally disordered or dangerous.

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