Unit 31: Introduction to Defenses


  • Dressler 193-210

  • Self-Defense Statutes (CP)

  • Peterson (CP)

Key Terms: 

  • failure of proof defense

  • justification

  • excuse

  • legal impossibility

  • renunciation

  • extrinsic defense



Criminal law defenses fall under two primary categories: justification and excuse.


A justification defense is a claim that, under the circumstances, the defendant’s act was socially acceptable and undeserving of liability. For example, shooting and killing someone is normally a crime. However, killing someone who is trying to kill an innocent third person provides a social benefit, and may be justified under the circumstances.

Justification defenses draw upon several theories. The “public benefit” theory holds that justification applies when the public benefits from commission of a normally prohibited act.  The “moral forfeiture” theory holds that while everyone normally has certain moral rights, these rights can be forfeited by certain voluntary actions. So in the example of the person killed in defense of another, a “moral forfeiture” argument would be that the would-be killer forfeited his right to life when he tried to kill an innocent victim, and the criminal law no longer recognizes a protectable interest in his life. The “moral rights” theory focuses on the right of a potential victim to defend her rights, and so would justify killing in self-defense on the grounds that the defendant had a moral right to defend herself. A final theory of justification is the “superior interest” theory, which holds that violations are justifiable when the actor is seeking to protect interests that are of greater value than the harm caused. For example, someone on the verge of starving to death who steals bread for necessary sustenance might be justified in her conduct, because the benefit of preserving her life outweighs the cost of the property theft.


Like justification defenses, excuse defenses hold that otherwise criminal conduct is, under the circumstances, not deserving of punishment. The two categories differ because justification focuses on the act, concluding that it was acceptable (or even desirable) under the circumstances, while excuse focuses on the actor, concluding that even though the act was undesirable, the actor should not be liable. For example, if a mentally ill defendant kills someone, the killing is socially undesirable (and therefore not justified) but might nonetheless be excused given the actor’s circumstances.

Several theories underlie the criminal law’s recognition of excuse defenses.  Under deterrence theory, if the act could not be deterred (e.g. it was done under coercion, or in a fit of insanity), then it should not be punished. Causation theory holds that an individual should only be blamed for conduct that was within his control. Character theory concludes that a person’s criminal conduct is excused if she has “good character;” that is, she did the wrong thing for the right reason. Finally, free choice theory excuses conduct that is committed without an opportunity (1) to understand the material facts of the situation, (2) to learn of the possible violation of law, or (3) to conform conduct with the law.

The difference between justification and excuse is important for at least two reasons. First, it sends a moral message to the public about what conduct we find socially acceptable, and what conduct we find merely excusable. Second, it can be practically important for third party liability. If conduct is justified (e.g., killing a would-be assassin), then aiding and abetting that conduct might also be justified. If conduct is merely excused (e.g., a mentally ill killer) then supplying a gun to the killer might be punishable even though the target offense was excused.

Other Defenses

Justification and excuse can be applied to any crime. Some defenses are crime-specific, however. For example, legal impossibility can be a defense to the crime of attempt, while renunciation can be a defense against attempt or conspiracy.

A final category of defenses is extrinsic defenses. These defenses do not challenge the culpability, wrongfulness, or guilt of the actor’s conduct. Rather, they rely upon some factor extrinsic to the crime bars conviction, or even prosecution. Examples of extrinsic defenses include statutes of limitations and diplomatic immunity.

Criminal law defenses generally place an obligation on the defendant to prove the defense by a preponderance of the evidence. “Failure of proof” defenses are an exception to this rule.  They are, in a sense, not really defenses but rather arguments that the prosecution has not met its burden of proof of an element of a crime.

Questions for Review:

Q1. Which justification theory do you find most persuasive, and why?

Q2. Which excuse theory do you find most persuasive, and why?

Q3. Do you think that a claim of self-defense is best seen as a justification or an excuse? Why?