The shot that forced the law to define insanity.
In 1843 a man fired at the wrong target on a London street. The killing that followed made judges write down, for the first time, exactly when a person is too ill to be guilty. Their answer still governs half of the United States.
A shot meant for the Prime Minister
On the afternoon of January 20, 1843, Edward Drummond was walking near Charing Cross in London, on his way toward Downing Street, when a man stepped up behind him and shot him in the back. Drummond was the private secretary to the Prime Minister, Sir Robert Peel. The gunman, a Glasgow woodturner named Daniel M'Naghten, was seized by a constable before he could fire a second pistol. Drummond seemed at first to have escaped the worst of it. He was able to walk home for treatment. Then complications set in, and he died five days later.
It is generally accepted, though never conclusively proved, that M'Naghten had meant to kill Peel and mistook Drummond for him. At trial the evidence painted a man in the grip of persecutory delusions: he believed he was being hounded by spies, followed everywhere, and driven to ruin by a conspiracy of his political enemies. What he did on that street was not, the defense argued, the act of a rational person choosing to kill. It was the act of a mind that no longer worked.
A verdict the country would not accept
M'Naghten stood trial at the Central Criminal Court, the Old Bailey, in March 1843, before the Lord Chief Justice of the Common Pleas, Sir Nicolas Conyngham Tindal. Medical witnesses described his delusions in detail, and the case for insanity was strong enough that the jury returned a verdict of not guilty on the ground of insanity. M'Naghten was not freed. He was committed to an asylum, where he spent the rest of his life. But he had escaped the gallows, and to a Victorian public that expected a man who shot another man dead to hang for it, that was a scandal.
The acquittal set off a public outcry. Newspapers attacked it, the establishment was alarmed, and Queen Victoria, who had herself been the target of more than one attempt, was among those who thought the defense of insanity had been stretched too far. The pressure fell on a question the common law had never squarely answered. Everyone agreed, in the abstract, that a person truly out of their mind should not be punished as a criminal. Almost no one could say precisely where that line fell, or how a jury was supposed to find it.
The questions the Lords asked
What happened next was unusual. The House of Lords invoked an old power to summon the judges of the common-law courts and put a set of abstract legal questions to them, not about M'Naghten's guilt, which was settled, but about the law of insanity in general. When should the defense succeed? What must a jury be told? The judges conferred and answered, and their answer, delivered by Chief Justice Tindal, with one judge responding separately, became one of the most quoted passages in the entire criminal law. It is remembered as the M'Naghten Rules.
Not guilty by insanity
M'Naghten was acquitted in 1843 on the ground of insanity, an outcome so unsettling to the public that it prompted the rules that bear his name.
Two ways to qualify
The test asks whether a disease of the mind left the accused unable to know the nature and quality of the act, or, if he did know, unable to know it was wrong.
Still the standard
A version of the M'Naghten test remains the insanity standard in about half of US states, and in England, more than 180 years later.
A modern limit
In 2020 the US Supreme Court held that no state is constitutionally required to offer the 'moral wrong' half of the test.
What the test actually asks
Stripped to its core, the rule says that to make out a defense of insanity it must be clearly proved that, at the time of the act, the accused was laboring under such a "defect of reason, from disease of the mind" that one of two things was true. Either he did not know the nature and quality of what he was doing, or, if he did know that, he "did not know he was doing what was wrong." Those two branches carry names that lawyers still use. The first is cognitive incapacity: the person does not understand the act itself, the classic example being someone so deluded he does not grasp that he is taking a human life. The second is moral incapacity: the person understands the act perfectly well but, because of illness, cannot tell that it is wrong.
The test does not ask whether the defendant was ill. It asks whether the illness reached one of two specific things: knowing what he was doing, or knowing that it was wrong.
That framing is narrower than it first sounds, and deliberately so. A person can be seriously, diagnosably mentally ill and still fail the M'Naghten test, because the test is not about the presence of illness but about its effect on knowledge at the moment of the crime. That narrowness is exactly what critics have pressed on ever since, and it is why other tests grew up alongside it.
The tests that grew up around it
Over the following century American courts experimented with alternatives, each trying to capture something M'Naghten seemed to leave out. Some jurisdictions added an "irresistible impulse" branch, asking whether illness had destroyed the defendant's power to control his conduct even if he knew it was wrong. New Hampshire, and later the federal courts in the District of Columbia, tried the broader "product" test associated with the case of Durham, which asked simply whether the crime was the product of mental disease. In 1962 the American Law Institute proposed a middle path in its Model Penal Code, excusing a defendant who lacked "substantial capacity" either to appreciate the wrongfulness of his conduct or to conform it to the law. For a time that formulation spread widely.
The pendulum then swung back. After the 1981 attempt on President Reagan's life ended in an insanity acquittal, Congress and many states tightened their standards, and a good deal of the country returned to something close to M'Naghten. Today a version of the M'Naghten test remains the insanity standard in roughly half of the states, and it is still the law in England and Wales. The 1843 answer to the Lords' questions turned out to be remarkably durable.
Where the line stands now
Durability is not universality. A small number of states have gone the other way and abolished the affirmative insanity defense altogether, allowing evidence of mental illness only to show that the defendant lacked the specific criminal intent, the mens rea, that the offense requires. Kansas is one of them. That approach reached the US Supreme Court in Kahler v. Kansas, decided in March 2020. James Kahler had been convicted and sentenced to death for killing four members of his family, and he argued that the Constitution's guarantee of due process required Kansas to let him raise the traditional insanity defense, including its moral-incapacity branch.
By a vote of six to three, the Court disagreed. Writing for the majority, Justice Elena Kagan held that the Due Process Clause does not force a state to build its insanity defense around the defendant's ability to know that his act was morally wrong. States, she wrote, retain broad room to define the relationship between mental illness and criminal responsibility, a question on which reasonable approaches have long differed. In dissent, Justice Stephen Breyer, joined by Justices Ruth Bader Ginsburg and Sonia Sotomayor, argued that eliminating the moral-incapacity inquiry abandoned a principle centuries deep in the common law. The split was not about whether M'Naghten is wise. It was about whether the Constitution makes any particular version of the insanity defense mandatory. The answer was no.
A rule that has organized this corner of the criminal law for nearly two centuries began with a single wrong turn on a London street.
Its central question has barely changed since 1843: not whether a defendant was ill, but whether the illness reached what he knew.
Archivar
The Archivar Editorial Desk
Researched and fact-checked against primary sources. General information for attorneys, not legal advice.
Sources: M'Naghten's Case (1843) 10 Clark & Finnelly 200, 8 English Reports 718; Kahler v. Kansas, 589 U.S. 271 (2020), via the US Supreme Court and Justia; the American Law Institute Model Penal Code (1962); the Legal Information Institute at Cornell Law School; and historical accounts of the case from the Wellcome Collection trial report, Psychiatric Times, and the UNC School of Government.
