The M’Naghten rule is a legal test for insanity established by the House of Lords in 1843. Under this rule, a defendant is presumed sane and responsible for their actions unless it is clearly proven that, at the time of committing the act, the defendant was suffering from a defect of reason due to a “disease of the mind” that either:
- Prevented the defendant from knowing the nature and quality of the act they were doing; or
- If the defendant did know what they were doing, they did not know that the act was wrong.
This rule focuses on the defendant’s cognitive ability to understand their actions and distinguish right from wrong. The burden of proof lies with the defendant to establish insanity on a balance of probabilities. If successfully proven, the verdict is typically “not guilty by reason of insanity,” which may lead to treatment instead of punishment.
In summary, legal insanity under the M’Naghten rule means that due to a mental illness or defect, the defendant either did not understand what they were doing or did not understand that it was wrong at the time of the crime.
