Define: M’Naghten Rules

M’Naghten Rules
M’Naghten Rules
Quick Summary of M’Naghten Rules

The M’Naghten Rules, also known as the M’Naghten test, are a legal standard used in common law jurisdictions, including the United Kingdom and some parts of the United States, to determine whether a defendant can be held criminally responsible for their actions due to insanity. The rules originated from the 1843 case of R v M’Naghten, where Daniel M’Naghten attempted to assassinate the British Prime Minister but was found not guilty by reason of insanity. According to the M’Naghten Rules, a defendant can be found not guilty by reason of insanity if, at the time of committing the act, they were suffering from a mental disorder that rendered them unable to understand the nature and quality of their actions or to distinguish right from wrong. The rules focus on the cognitive capacity of the defendant at the time of the offense and do not take into account emotional or volitional factors. The M’Naghten Rules have been influential in shaping the legal approach to insanity defenses in many common law jurisdictions.

Full Definition Of M’Naghten Rules

The standard rules applied to the determination of whether a person who offers an Insanity defence was insane at the time of the commission of the offence. The Rules derive from a debate in the House of Lords concerning the outcome of Mnaghtens case (1843). The relevant passage is …it must be clearly proved that, at the time of committing the act, the party accused was labouring under such a defect of reason, from 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.

Analyzed in detail, this statement says that the accused must be suffering from a ‘disease of the mind’, and the disease gave rise to a ‘defect of reason’, and he was not aware of the nature of the act did not know he was doing wrong.

Disease of the mind A ‘disease of the mind’ need not be a disease of the brain, or even a psychiatric condition: arteriosclerosis, epilepsy and diabetes have all be held to be forms of insanity. The rationale is that all have an effect on the ‘mind’, and all result from internal factors. On the contrary, a blow to the head resulting in mental derangement will probably lead to acts classifiable as Automatism rather than insanity. Although there is no logical or medical basis for this, it may have the practical benefit that ‘internal’ factors are liable to recur, and therefore require detention for treatment.

Defect of reason The disease of the mind must interfere with the process of reasoning or understanding. The defence does not exist for a person who simply fails to think about his actions, perhaps as a result of depression.

Nature of the act The accused must show that he was unaware of the nature of the act, not the moral or legal nature. A person who believes he was not doing wrong may, however, escape liability under the other limb of the M’Naghten test (as described below). A person who kills someone under the delusion that he is on a battlefield may escape liability in this way. A person who is so deluded that he stabs someone thinking he is cutting down a tree may also escape, but he also lacks the Mens Rea of murder, so may not be liable anyway.

Whether the accused knew he was doing ‘wrong’ If the accused knew that the act was illegal, then he knew he was ‘doing wrong’ for the purpose of this test. In other words, legal wrongness, not moral wrongness, is important.

Note that an irresistible impulse to commit a crime cannot found a defence of insanity, however strong.

Related Phrases
No related content found.
Disclaimer

This site contains general legal information but does not constitute professional legal advice for your particular situation. Persuing this glossary does not create an attorney-client or legal adviser relationship. If you have specific questions, please consult a qualified attorney licensed in your jurisdiction.

This glossary post was last updated: 28th March, 2024.

Cite Term

To help you cite our definitions in your bibliography, here is the proper citation layout for the three major formatting styles, with all of the relevant information filled in.

  • Page URL:https://dlssolicitors.com/define/mnaghten-rules/
  • Modern Language Association (MLA):M’Naghten Rules. dlssolicitors.com. DLS Solicitors. March 28, 2024 https://dlssolicitors.com/define/mnaghten-rules/.
  • Chicago Manual of Style (CMS):M’Naghten Rules. dlssolicitors.com. DLS Solicitors. https://dlssolicitors.com/define/mnaghten-rules/ (accessed: March 28, 2024).
  • American Psychological Association (APA):M’Naghten Rules. dlssolicitors.com. Retrieved March 28, 2024, from dlssolicitors.com website: https://dlssolicitors.com/define/mnaghten-rules/