M'Naghten Rules

The M'Naghten Rules set the legal test applied to William Chester Minor in 1872: whether a defendant was of unsound mind and had killed while under a delusion. They distinguished criminal responsibility from the fact of the killing, allowing the jury to return an insanity verdict while leaving the judge to decide what custody should follow.

Origin and Legal Purpose

For thirty years before Minor’s trial, the rules had guided decisions in cases involving insanity. They took their name from a man who, in 1843, shot dead the private secretary to Sir Robert Peel and was acquitted because his madness meant he could not tell right from wrong. The rules were concerned not simply with whether a killing had occurred, but with whether the accused could be held criminally responsible for it.

The Test Applied to Minor

At Minor’s trial, the Chief Justice instructed the jury to apply the rules to the evidence of delusion they had heard. If they were convinced that Minor was of unsound mind and had killed George Merrett while under a delusion of that kind, they were to find him not guilty on grounds of insanity. The judge, rather than the jury, would then determine the necessary custodial measure.

Verdict and Consequence

The jury reached its decision without deliberation on the afternoon of 6 April 1872. Its verdict treated Minor as legally innocent by reason of insanity, although the court and Minor knew that he had killed Merrett. The distinction built into the rules thus produced a consequential split: the verdict did not absolve Minor from custody. The judge ordered him held in safe custody until the Crown determined what should happen next, beginning an indefinite confinement under the authority of the law.