## Facts

- Daniel M’Naghten, a Scottish wood-turner, shot Edward Drummond near Downing Street in Middlesex on January 20, 1843; Drummond died five days later.
- Evidence indicated M’Naghten suffered paranoid delusions that religious and political groups were persecuting him and conspiring to kill him.
- Medical testimony stated a person may have “morbid delusions” and generally understand right and wrong, yet be driven by the delusion in relation to acts connected to it.
- M’Naghten was indicted for willful murder at the Central Criminal Court and pleaded not guilty by reason of insanity.
- The trial judge directed the jury to consider whether M’Naghten had sufficient understanding to know he was doing a wrong or wicked act and violating the laws of God and man.
- The jury returned a verdict of not guilty on the ground of insanity.
- Public controversy followed, and the House of Lords requested the common law judges’ opinions on abstract questions concerning criminal responsibility where insanity and delusion are alleged.

## Issues

1. What legal standard governs criminal responsibility when the accused acts under insane delusion.
2. What jury directions apply regarding the presumption of sanity and the proof required to establish insanity.
3. How “partial delusion” affects responsibility for a criminal act.
4. Whether, and in what manner, medical experts may give opinion evidence on the accused’s capacity to distinguish right from wrong.

## Decision

- The judges advised that every person is presumed sane and responsible unless insanity is proved to the jury’s satisfaction.
- Insanity is established only if it is clearly proved that, at the time of the act, the accused had a defect of reason from disease of the mind such that the accused did not know the nature and quality of the act, or did not know the act was wrong.
- When an accused acts under a delusion, responsibility is assessed as if the delusional facts were real.
- A person acting under an insane delusion remains punishable if the person knew, at the time, that the conduct was contrary to law.
- A medical witness who heard the trial evidence may be asked, as a matter of science, whether the assumed facts indicate a mental state incapable of distinguishing right from wrong.

## Legal Principles

- Sanity is presumed; the party asserting insanity bears the burden to prove it to the jury’s satisfaction.
- The insanity defense is a cognitive test: the relevant inquiry is knowledge and understanding, not inability to control conduct.
- The required showing is a defect of reason caused by disease of the mind that prevents knowledge of (a) the nature and quality of the act, or (b) that the act is wrong.
- Delusion does not automatically excuse; the accused is judged as if the delusional beliefs were true, and liability turns on whether the act would be criminal on those assumed facts.
- If the accused knew the act was contrary to law, the accused is punishable notwithstanding a delusional motive of revenge, redress, or perceived public benefit.
- Expert opinion may be elicited through hypothetical questions based on facts in evidence to address capacity to distinguish right from wrong.

## Conclusion

M’Naghten’s Case supplied the common law’s leading formulation of the insanity defense by requiring clear proof of a disease-of-mind defect that negates the accused’s knowledge of the act’s nature or its wrongfulness, while treating delusional defendants as if their supposed facts were true and maintaining punishment where the accused knew the conduct was unlawful.


M’Naghten’s Case (1843) 10 Cl. & Fin. 200 (H.L.)