Criminal Law


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.