M'Naghten Rule Meaning and Definition

M'Naghten Rule Legal Definition

A test that is applied to determine whether a person that is accused of a crime was sane at the time of its commission. If so, that person is criminally responsible for the wrongdoing.

A defendant is entitled to an acquital if proof establishes:

1. A mental disease (disease of the mind)
2. Caused a defect of reason
3. such that the defendant lacked the ability at the time of her actions to either:

a. know the wrongfulness of her actions; or
b. understand the nature and quality of her actions.

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The M'Naghten rule is a test that is used for criminal insanity. According to the M'Naghten rule, a criminal defendant is not guilty, by reason of insanity if, at the time of the alleged criminal act, the defendant was deranged in a way that he/she did not know the quality or nature of his/her actions or, if he/she knew of the quality and nature of his/her actions, he/she was so deranged that he/she did not know that what he/she was doing was wrong.

The M'Naghten rule is named for Daniel M'Naghten. In 1843, Mr. M'Naghten tried to kill England's prime minister, Sir Robert Peel. M'Naghten had thought Peel wanted to kill him, so he tried to shoot Peel, however, he instead shot and killed Peel's secretary, Edward Drummond. At Mr. M'Naghten's trial medical experts testified that M'Naghten was psychotic, and M'Naghten was found not guilty by reason of insanity.

The public were outraged at the verdict, and the House of Lords in Parliament ordered the Lords of Justice of the Queen's Bench to create a strict definition of criminal insanity. The Lords complied and declared that insanity was a defense to criminal charges only if:

"...at the time of the committing of the act, the party accused was labouring under such a defect of reason, from a disease of the mind, as not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what was wrong." (Queen v. M'Naghten, 8 Eng. Rep. 718 [1843])

The aim of the M'Naghten rule was to limit the Insanity Defense to cognitive insanity, which is a basic inability to distinguish right and wrong. Other tests that have been formulated by courts and legislatures since M'Naghten have supplemented the M'Naghten rule with another form of insanity known as volitional insanity.

Volitional insanity is experienced by mentally healthy persons who, although they understand that what they are doing is wrong, they are so mentally unbalanced at the time of the criminal act that they are not able to conform their actions to the law. The M'Naghten rule was adopted in most jurisdictions in the United States, however courts as well was legislatures eventually modified and expanded its definition.

The definition of criminal insanity now varies jurisdictionally, but most jurisdictions have been influenced by the M'Naghten rule.

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