Affirmative Defense Essay

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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 rea). 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.

Considerable dispute exists about the rationale for the excusing affirmative defenses, but most depend on a finding that the defendant was not capable of rationality or that the defendant was compelled to act. Legal insanity is an example of the former; duress is an example of the latter. An important question is whether the law should establish new affirmative defenses of excuse for newly discovered variables, such as new mental syndromes or brain abnormalities that seem to play a causal role in criminal behavior. Advocates argue for such excuses, but causation alone is not an excusing condition. At most, such causes can support the existence or expansion of a genuine existing excuse, such as legal insanity.

Although the distinction between justifications and excuses can be stated clearly, it can be very blurry. For example, suppose a homicide defendant actually believed that he was in deadly danger, but he made a reasonable mistake and was not in danger at all. Has this person done the right thing, or was it wrong but he was not responsible? For another example, suppose the defendant kills for no justifying reason, but he would have been justified if he knew all the facts, such as that the victim had a hidden weapon and was about to kill the defendant wrongfully. Justification or excuse? On the one hand, the defendant’s conduct was “objectively” justified, but, on the other, the defendant subjectively acted for a nonjustified reason.

Such questions divide criminal lawyers and raise important theoretical and practical issues about culpability. Justified and excused defendants are both found not guilty, but the former have done the right thing; the latter have done the wrong thing and, in some cases, may still be dangerous. The criminal law is a teacher that educates and guides citizens. It should therefore announce clearly what behavior is right or permissible and what is wrong and forbidden. Defendants also care about whether their conduct is justified or excused because any defendant would prefer to have his or her harmful conduct authoritatively labeled right, rather than wrong but excused, and justifications do not trigger state control.

Policy affirmative defenses do not negate the defendant’s blameworthiness, but permit exoneration for other good reasons. The statute of limitations and diplomatic immunity, for example, bar conviction of a defendant who has undoubtedly committed a crime. The State concludes, respectively, that the defendant may not be able to defend himself adequately after a certain period of time, or that good international relations require that we not convict the diplomats of other nations residing in the United States.


  1. Dressler, Joshua. 2006. Understanding Criminal Law. 4th ed.
  2. Dayton, OH: Matthew Bender. Greenawalt, Kent. 1984. “The Perplexing Borders of Justification and Excuse.” Columbia Law Review 84:1897.
  3. Morse, Stephen J. 1998. “Excusing and the New Excuse Defenses: A Legal and Conceptual Review.” Crime and Justice 23:329-406.
  4. Morse, Stephen J. 2002. “Uncontrollable Urges and Irrational People.” Virginia Law Review 88:1025-78.

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