Criminal Defenses

Introduction

Criminal defenses are legal arguments that, if successful, relieve a defendant of criminal liability. Defenses may be categorized as justifications (conduct that is not wrongful because it serves a socially beneficial purpose) or excuses (conduct that is wrongful but the actor is not blameworthy due to some personal condition or circumstance). The principal defenses include self-defense, insanity, entrapment, necessity, and duress.

Self-Defense

Self-defense justifies the use of force against another person to prevent imminent unlawful harm. Under the common law and the MPC § 3.04, a person may use nondeadly force if they reasonably believe it is necessary to protect against the imminent use of unlawful force. Deadly force is justified only when the actor reasonably believes it is necessary to protect against death, serious bodily injury, or certain forcible felonies.

The reasonableness requirement is evaluated from the perspective of a reasonable person in the defendant’s circumstances. The retreat rule requires a person to retreat before using deadly force if they can do so with complete safety, though many states have abolished the retreat rule through stand your ground laws. The castle doctrine provides that a person has no duty to retreat from their home before using deadly force.

Insanity

The insanity defense excuses criminal conduct when a mental disease or defect prevents the defendant from understanding the nature or wrongfulness of their actions. Several tests exist. The M’Naghten test requires that the defendant did not know the nature and quality of the act or did not know that it was wrong. The irresistible impulse test adds a volitional component: even if the defendant knew the act was wrong, they could not control their conduct.

The Model Penal Code test provides that a person is not responsible if, as a result of mental disease or defect, they lack substantial capacity either to appreciate the criminality of their conduct or to conform their conduct to the requirements of law. The federal insanity defense (18 USC § 17) requires the defendant to prove by clear and convincing evidence that they lacked the capacity to appreciate the wrongfulness of their conduct.

Entrapment

Entrapment occurs when the government induces a person to commit a crime that the person was not otherwise predisposed to commit. The subjective test, adopted by federal courts and a majority of states, focuses on the defendant’s predisposition. If the defendant was predisposed to commit the crime, there is no entrapment, even if the government provided an opportunity or used persuasion.

The objective test, adopted by some states and the MPC § 2.13, focuses on the government’s conduct. If the government employed methods of persuasion that created a substantial risk that a person not predisposed would commit the crime, the defense is established. The objective test is designed to deter improper police conduct rather than to assess the defendant’s culpability.

Necessity

Necessity (also called choice of evils) justifies conduct that causes harm but avoids a greater harm. Under the MPC § 3.02, conduct is justified if the actor believes it is necessary to avoid a harm or evil to themselves or another, and the harm or evil sought to be avoided is greater than that sought to be prevented by the law defining the offense.

The necessity defense is available only when the defendant did not create the emergency, the legislature has not already balanced the competing values, and no adequate legal alternative exists. Necessity is not available for homicide, as the law does not permit balancing one life against another.

Duress

Duress excuses criminal conduct when the defendant acts under the threat of imminent death or serious bodily injury. Under the MPC § 2.09, duress is a defense if the defendant was coerced to commit the offense by the use of or threat of unlawful force against the defendant or another, and a person of reasonable firmness would have been unable to resist.

Duress is not available for the offense of homicide in most jurisdictions. The defense also requires that the threat be imminent and that the defendant have no reasonable escape. Duress is an excuse, not a justification: the conduct is wrongful, but the defendant is not blameworthy because they acted under overwhelming pressure.

Conclusion

Criminal defenses reflect fundamental judgments about when conduct is justified or excused. Self-defense and necessity justify otherwise harmful conduct, while insanity and duress excuse the actor despite the wrongfulness of the conduct. The entrapment defense limits the government’s role in manufacturing crime. Each defense raises complex questions about individual responsibility, social utility, and the limits of criminal punishment.