Criminal Law



Self-defense is a justification defense when a non-aggressor uses force to defend himself. It requires an honest and reasonable belief that the use of force was necessary to defend against the immediate use of unlawful force and that the force used was reasonably necessary in the circumstances.

The use of force must not be excessive in relation to the harm threatened.

At common law, deadly force is never permitted to repel a non-deadly attack, nor may it be used if a non-deadly response will apparently suffice. The Model Penal Code allows it to protect against immediate death, serious bodily injury, forcible rape, or kidnapping.

The Model Penal Code instead requires that the force be "immediately necessary" to permit force when required to repel the attack instead of when the threat is immediate.

At common law, a person is required to retreat if he can do so in complete safely. However, a slight majority of jurisdictions have enacted stand your ground laws, removing the duty to retreat. In addition, all jurisdictions have an exception to the duty to retreat if one is attacked in his own dwelling place or it curtilage (immediately surrounding land). Some jurisdictions expand this to one's vehicle, and the MPC expands it to one's place of work.


Necessity is a justification defense when one commits a relatively minor offense to avoid suffering or allowing another to suffer a substantial harm to person or property.

Elements of necessity:

  1. The defendant must be faced with a clear and imminent danger.
  2. The defendant must have a reasonable expectation that his action will be effective in abating the danger he seeks to avoid.
    • There must be a direct causal relationship between his action and the harm to be averted.
  3. There must be no effective legal way to avert the harm.
  4. The harm that the defendant will cause by violating the law must be less serious than the harm he seeks to avoid.
    • The defendant’s actions should be weighed against the harm reasonably foreseeable at the time, rather than the harm that actually occurs.
  5. Lawmakers must not have previously “anticipated the choice of evils” and determined the balance to be struck in a manner which conflicts with the defendant’s choice.
  6. The defendant must come to the situation with “clean hands”. He must not have substantially contributed to the emergency.

Some states limit the defense to emergencies created by natural, non-human forces.

Necessity cannot apply to homicide cases.

Some courts only allow necessity to protect people and property.


Duress is an excuse defense when one reasonable believes that committing a crime is:

  1. His only way of rebuffing a
  2. Credible and
  3. Imminent
  4. Threat of death or grievous bodily injury to himself or a member of his family,
  5. While not being at fault for creating the coercive situation.

A crime committed under duress is committed to further the criminal activity of the aggressor, not to resist him.


Intoxication is sometimes a defense.

Neither voluntary nor involuntary were valid defenses at common law.

While voluntary intoxication is still inadmissable as evidence in general intent crimes, many jurisdictions allow it to negate the mens rea element of specific intent crimes.

  • It is not constitutionally required that it ever be allowed as a defense however.

Involuntary intoxication is a complete defense if it causes the defendant to become unable to form the requisite mens rea.


Insanity is an excuse defense when one is too insane to warrant criminal punishment.

There are three competing tests for determining insanity:

M'Naghten Test

The M'Naghten test, also known as the cognitive test, says that a person is not guilty by reason of insanity when he, at the time he committed the act:

  1. Had a mental defect
  2. Which either:
    1. Prevented him from knowing the nature and quality of the act he was doing or
    2. From knowing that what he was doing was wrong.
      • Wrongfulness

        Wrongfulness can be understood in one of two ways:

        1. Illegal
        2. Morally wrong
          • Moral wrongfulness can also be determined subjectively or objectively:
            • The objective standard asks whether the actor understood his actions violated society's view of morality.
            • The subjective test asks whether the actor was able to understand that his actions violated his own personal moral code.
Irresistible Impulse Test

Some jurisdictions have established the irresistible impulse test to broaden the M'Naghten test by providing a third prong under which to find a defendant insane—irresistible impulse.

This says that, as an alternative to not knowing the nature and quality of his act or that it was wrong, one may also be found insane if he suffers from a volitional defect that prevents him from complying with the demands of the law.

Model Penal Code Test

The Model Penal Code's "substantial capacity" test includes the irresistible impulse test (and therefore the M'Naghten Test), but further broadens the doctrine by allowing the insanity defence when a criminal retained some capability to under or control his criminal behavior.

This test only requires that a defendant lack the substantial capacity to appreciate the criminal of his conduct or to conform his conduct to the requirements of the law.