Case Analysis : R v McNaughten M’Naghten

R v M’Naghten (1843) 8 E.R. 718; (1843) 10 Cl. & F.



In January 1843, at the parish of Saint Martin, Middlesex, Daniel M’Naghten took a pistol and shot Edward Drummond, who he believed to the British Prime Minister Robert Pell, wounding him fatally. Drummond died five days later and M’Naghten was charged with his murder. He pleaded not guilty by reason of insanity.

At trial, evidence was given of the shooting of Drummond and witnesses were called on the behalf of the defendant, M’Naghten, to attest to the fact he was not in a sound state of mind at the time of committing the act. Some of the witnesses who gave this evidence, had previously examined M’Naghten, whilst others had not seen him prior to the trial and, and they formed their opinion on hearing the evidence given by other witnesses.

The medical evidence brought forward stated that persons of otherwise sound mind, might be affected by morbid delusions and that M’Naghten was so affected. A person labouring under such delusion, might usually possess a moral perception of right and wrong, but in relation to acts connected to their delusion may be carried beyond power of their own control leaving them with no such perception.

Accordingly M’Naghten was not capable of exercising control over his acts whilst under his delusion. Due to the nature of M’Naghten’s condition these delusions went on gradually until they reached a climax, ending with Drummond being shot. Evidence brought before the Court about the condition from which M’Naghten suffered stated that a man may go on for years quietly whilst under the delusion’s influence, but had the potential break out into extravagant and violent paroxysms.

In relation to the charge against M’Naghten, Lord Chief Justice Tindal stated that “the question to be determined is, whether at the time the act in question was committed, the prisoner had or had not the use of his understanding, so as to know that he was doing a wrong or wicked act. If the jurors should be of opinion that the prisoner was not sensible, at the time he committed it, that he was violating the laws both of God and man, then he would be entitled to a verdict in his favour: but if, on the contrary, they were of opinion that when he committed the act he was in a sound state of mind, then their verdict must be against him.”

M’Naghten was found not guilty.

Following this a panel of Judges attended the House of Lords and had a series of hypothetical questions on the topic of insanity put before them.


The hypothetical questions about insanity the judges had to address were as follows:

  1. What is the law respecting alleged crimes committed by persons afflicted with insane delusion, in respect of one or more particular subjects or persons: as, for instance, where at the time of the commission of the alleged crime, the accused knew he was acting contrary to law, but did the act complained of with a view, under the influence of insane delusion, of redressing or revenging some supposed grievance or injury, or of producing some supposed public benefit?
  2. What are the proper questions to be submitted to the jury, when a person alleged to be afflicted with insane delusion respecting one or more particular subjects or persons, is charged with the commission of a crime (murder, for example), and insanity is set up as a defence?
  3. In what terms ought the question to be left to the jury, as to the prisoner’s state of mind at the time when the act was committed?
  4. If a person under an insane delusion as to existing facts, commits an offence in consequence thereof, is he thereby excused?
  5. Can a medical man conversant with the disease of insanity, who never saw the prisoner previously to the trial, but who was present during the whole trial and the examination of all the witnesses, be asked his opinion as to the state of the prisoner’s mind at the time of the commission of the alleged crime, or his opinion whether the prisoner was conscious at the time of doing the act, that he was acting contrary to law, or whether he was labouring under any and what delusion at the time?


In response to these questions the Judges formulated the M’Naghten Rules (1843) 4 St.Tr.(N.S.) 847. These provide the legal definition of insanity. They provide that a defendant wishing to rely on the defence of insanity must show that:

  1. They laboured under a defect of reason
  2. Caused by a disease of the mind; so that either
  3. He did not know the nature and quality of his acts, or that he did not know what he was doing was wrong.

Lord Chief Justice Tindal delivered the opinion for the House of Lords. Jurors should be instructed that every man is presumed sane and to possess a sufficient degree of reason to be responsible for his crimes. Therefore, in order to establish an insanity defense, it must be clearly proven that at the time of the act, the accused was under such a defect of reason from disease of the mind that he did not know the nature and quality of the act he was committing; or if he did know, he did not know what he was doing was wrong.

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