Duhaime's Law Dictionary

M'Naghten Rules Definition:

A defence to criminal law liability developed in England; if, at the time of the offence, the accused had a disease of the mind such that he was unable to know that his act was wrong.

Related Terms: Dementia, Insanity, Lunacy law, Mens Rea

An insanity defence rule in criminal law developed in England in the mid-1800s, available if, at the time of the offense, the accused was insane, had a disease of the mind such that he was unable to know that his act was wrong.

The much-cited words of the judgment as reported in the official law reports of the House of Lords as now reported in the 8th volume of the English Reports:

"(T)he jurors ought to be told in all cases that every man is to be presumed to be sane, and to possess a sufficient degree of reason to be responsible for his crimes, until the contrary be proved to their satisfaction; and that, to establish a defence on the ground of insanity, it must be clearly proved that at the time of the committing of 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."

Mental disorderAlso sometimes referred to as:

  • MacNagthen’s Case (although the proper spelling is indeed “M’Nagthen”);
  • The Rules in M’Nagthen’s Case;
  • M’Naghten Rule;
  • simply "M’Naghten;";
  • M’Naghten Standard for Insanity (McKinney v State);
  • M’Nagthen Test;
  • M’Naghten Test of Insanity; and even
  • M’Naghten Test of Legal Insanity.

In 1843, Daniel M’Nagthen was out to assasinate the England Prime Minister Robert Peel but instead, shot and killed his assistant, Edward Drummond. M'Naghten was acquitted by reason of insanity and the public outcry was so great, that a rule was developed to handle such future claims.

For a time, the M’Naghten’s Rules served as the standard against which criminal responsibility was judged. Actually, most jurisdictions adopted by statute the principle that an individual suffering from mental disorder making him or her incapable of appreciating his or her act(s) is not criminally responsible.

Eventually, even England decided to no longer rely on the M'Nagthen rules and brought in statute law on this topic. For example, Chapter 3, §3 of the Penal Code of 1889 and, previously, the Trial of Lunatics Act 46 & 47, Vict. Chapter 38. all, though, were inspired by the wisdom of the M'Naghten’s Rules.

When first published, the M'Nagther’s case shook the common law but very quickly became the law throughout the common-law world. In fact, of any shortlist of influential English legal decisions but have to appear this case. It is often used as a reference to those who would propose that the common-law, as distinguished from other systems of law, is a kinder gentler system. The legal literature of the time expounded considerably upon this decision.

This 1843 legal decision of the House of Lords has been made available to our users in PDF form: click here to view that document. The facts are that Daniel M’Naughten shot and killed Edward Drummond on January 20, 1843. M’Naghten was known to be "off" to those that knew him and to many, had to be acquitted of murder on the ground of insanity.

Unfortunately, and certainly contrary to modern legal decisions, the description of the murder and in particular of his state of mind or so-called insanity, even in the Court's judgment is deficient. According to the House of Lords decision, the bare facts were that:

“… the prisoner (M’Naghten) … was not, at the time of committing the act, in the sound state of mind.”

Further, that he may have been:

“…. laboring under a morbid delusion … A delusion which carried him away beyond the power of his own control … and that he was not capable of exercising any control over acts…. that it was of the nature of the disease with which the prisoner was affected to go on gradually until it had reached a climax, when it burst forth with irresistible intensity; but I'm man might go on for years quietly, Though at the same time under its influence, but with all that once break out into the most extravagant and violent paroxysms.”

It is not possible with any degree of certainty to reverse engineer that brief description to suggest any particular mental disorder but it does have some characteristics of what we might now diagnose as schizophrenia.

The bumper sticker version f the case has often been stated that the test in terms of criminal responsibility and state of mind is that the accused be in possession of the faculty of distinguishing right from wrong1 and cannot otherwise be convicted.

The British Trial Of Lunatics Act of 1883 provided that:

"Where in any indictment or information any act or omission is charged against any person as an offence, and it is given in evidence on the trial of such person for that offence that he was insane, so as not to be responsible, according to law, for his actions at the time when the act was done or omission made, then, if it appears to the jury before whom such person is tried that he did the act or made the omission charged, but was insane as aforesaid at the time when he did or made the same, the jury shall return a special verdict that the accused is not guilty by reason of insanity."

M'Naghten's "test" or "rule" was subsequently adopted by many other common law jurisdictions. In Greiman v Thalacker, Justice Arnold of the United States Court of Appeals wrote:

"... the M'Naghten Rule, according to which no person can be convicted of a crime if he or she suffers from a diseased mind that renders him or her incapable of knowing the nature and quality of the act that he or she is committing, or of distinguishing between right and wrong in relation to that act."

Also, Justice Boochever of the Supreme Court of Alaska in McKinney v State:

"Under ... the M'Naghten standard for insanity ... it must be clearly proved that, at the time of committing the act, the party accused was laboring 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 and of distinguishing between right and wrong in relation to the act with which he is charged."

It was also summarized in this 1990 decision of Canada's Supreme Court, Chaulk v the Queen (aka R v Chaulk):

"The central proposition in the M'Naghten Rules was that the defence would be available to someone who, because of a defect of reason resulting from disease of the mind, did not know the nature and quality of the act he was doing; or if he did know it, he did not know he was doing what was wrong.

"The first alternative deals with the situation where insanity negatives mental elements in the definitions of offences."The second alternative establishes a special exculpatory defence which is based on lack of capacity for normative understanding

"There is, in effect, an exception to the general rule that ignorance of the law is no excuse.  The third way in which insanity could be relevant to criminal culpability is through a cognitive breakdown leading to a mistaken belief in a matter of contextual permission. This situation was covered in the M'Naghten Rules by a ruling that responsibility would be determined as if the facts were as they were believed to be.”


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