2.6 M’Naghten’s Case

2.5 Not Guilty by Reason of Insanity

Book Two / 6

The availability under the law of an insanity defense is the result of a long history of social and judicial development.  Central to the debates over the decades have been the complicated doctrine of criminal intent, the presumption of a defendant’s sanity, and the amount of mental illness sufficient to overcome that presumption.

All three of these concepts have been the subject of legal argument for centuries.  Case law defining insanity and refining its application to criminal defendants has developed steadily in both England and the United States since the mid-nineteenth century.

The quality of these judicial opinions reflects the sobriety with which the courts have approached the issue of criminal insanity.  Their quantity suggests how frequently the issue has arisen since its most famous appearance in a court of law.


It was the spring of 1843, a few years before the 1848 revolutions would shake the European continent.  The British House of Lords was convening for an inquiry into issues arising from the acquittal of Daniel M’Naghten of the murder of Edward Drummond.

Edward Drummond had been the Secretary to the British Prime Minister, Sir Robert Peel, against whose government a number of citizens had been pressing for broader franchise.  By some reports Daniel M’Naghten had worked with one of the more prominent leaders of that movement.

On a January day that year, M’Naghten — possibly believing that Edward Drummond was the prime minister himself — followed Drummond as he walked toward Downing Street and shot him in the back.  Drummond died from the wound a short time later.

At the subsequent trial M’Naghten was defended by a well-known barrister who had read a recently published treatise about insanity and the criminal law.  The barrister focused his defense of his client on the man’s state of mind at the time of the shooting.  He attempted to convince the jurors that although they might find that M’Naghten had committed the act, they could not hold him legally responsible due to a mental defect he had suffered at the time.

The defense called expert witnesses at the trial to provide medical evidence of M’Naghten’s state of mind on the occasion of the shooting.  Some of these experts had examined M’Naghten prior to trial.  Others had never met him before.  Nevertheless, the judge allowed each expert to testify on his professional opinion of M’Naghten’s mental state at the time of the shooting.  The experts were permitted to base their opinions solely on the testimony of witnesses at trial who had seen the shooting and observed M’Naghten’s conduct.

Upon conclusion of the trial the presiding judge drafted the question that would guide the jurors’ deliberation on the defendant’s guilt or innocence.  As reported in the digest of a later inquiry in the House of Lords, the judge framed the question for the jury to answer in words that would soon find their way across the Atlantic and into the statutes and case law of a newer nation:

“Whether at the time the act in question was committed, the prisoner had or had not the use of his understanding, so as to know that he was doing a wrong or wicked act.”

After the judge instructed the jurors on the law, he sent them to deliberate on the factual evidence, on the medical opinions provided, and on the jurors’ own observations of the defendant at trial.  The verdict returned was “not guilty on the ground of insanity.”

The verdict was stunning.  In response, Queen Victoria requested that the House of Lords review the case.  In turn, the House of Lords invited twelve justices from the common law courts to provide their opinions.  The entire group assembled on two occasions to discuss the state of the law controlling a plea of insanity in a criminal prosecution.


Although each generation since M’Naghten’s acquittal has painted its unique gloss of popular understanding upon the defense of insanity, the fundamental questions debated by British society in 1843 remain largely the same today.  Those questions, as they were presented to the twelve justices in the House of Lords that year, were substantially the following:

  1. If an individual is under an insane delusion and commits an offense as a consequence of that delusion, is he excused from criminal liability as a result.
  2. If an expert in the area of insanity has never seen the defendant prior to trial but is present throughout the trial, can he testify on the defendant’s state of mind at the time of the commission of the act?  Can he testify about whether the defendant knew that he was acting contrary to law at the time of the act, or was instead acting under a delusion?
  3. Finally, what questions are to be given to a jury when the defendant claims insanity as a defense?  What terms should be used regarding the defendant’s state of mind at the time of the commission of the act?

The questions are striking in that they reveal to us the apparent novelty of the ideas confronted by the legislators in mid-nineteenth century England concerning an issue that is well-known to us now.  The possibility that a jury might find that a man had committed a violent act — perhaps even the murder of a prominent public servant — but was not to be held legally to blame for his conduct due to his mental illness has become familiar, if not comfortable, to our contemporary courts.  At the time of M’Naghten’s trial, however, it seemed unimaginable to most.

Perhaps the verdict had been a fluke.  It may have been merely the fortunate result of hiring an intelligent attorney who kept himself cross-informed on the literature arising from other academic arenas.  But it may also have been an early illustration of an aspect of the modern era that was beginning to find its voice in the law and in the power of a deliberative jury.  The verdict might instruct us on one way through which a society’s understanding of itself begins to mature, and then gifts its institutions with a new take on an old idea, demanding — almost through the sheer shock of it all — that those institutions listen.  Through the filter of a defendant’s peers, combined as a jury in a court of law, the British House of Lords had itself received a summons of sorts, from the people, to consider society’s responsibility to the mentally ill criminal.

Whether a result of the jurors’ long-simmering frustrations or simply the product of a crafty litigator, the verdict was a wake-up call to British legislators.  It asked the State to consider more closely the difference between violent crimes committed by the sane citizen and those committed by the insane.  Something was very different about the two according to this one British jury in the mid-nineteenth century.  Perhaps they could not define the difference if asked to explain it, but they could use the event of an historical assassination to call attention to its existence.

The justices had appeared before the House of Lords that day with humility and concern, recognizing the traps inherent in pronouncing a set of general rules that would be applied prospectively to the prosecution of thousands of different defendants, each with a unique profile of mental illness.

One justice, as if he could foresee the volumes of future case law that would quote his exact words, determined to conclude his introductory remarks with an apology to future jurists, acknowledging frankly that the rules in their advisory opinion that day might prove “as little satisfactory to others as they are to myself.”

Nevertheless, in the process of their debate and decisions on the questions before them that year, the justices established a template for the insanity defense that would guide not only British but American courts for years to come.  And, as predicted, that template would prove as little satisfactory to this country’s courts as they feared it would be to those of its origin.


As the one British justice anticipated, few words in the development of criminal law have been as prescient as his own.  The proceedings that year in the House of Lords became known as M’Naghten’s Case, and the justices’ answers to the question of whether an insane condition may excuse a defendant from criminal responsibility was termed M’Naghten’s Rule.

M’Naghten’s Rule appears simple at first glance.  One justice addressing the House of Lords had summarized it this way:  an accused is not punishable if at the time of the act an unsoundness of mind rendered him incapable of knowing right from wrong.  A second justice, pronouncing the inverse of the same rule, explained that one who is not otherwise insane, but experiences a partial delusion only, is nevertheless punishable if at the time he committed the crime he knew “he was acting contrary to law.”

On account of the rule’s reliance on the defendant’s cognitive awareness of the nature of his acts, M’Naghten’s rule would come to be known as the knowledge test.  The test ruled out all defendants who knew at a conscious level that their actions were wrong, even if their illness corrupted their capacity for self-control.

As a result, the availability and success of the defense was limited to a very small minority whose illness obliterated any present perception of the reality of the world around them.  Defendants whose illness was severe enough to interfere with their self-control, but who nonetheless knew at the time that what they were doing was wrong, these defendants could still be held criminally accountable in court, and sentenced to the harsh conditions of incarceration without treatment.

M’Naghten’s Rule swiftly crossed the Atlantic.  It appeared in case law of the United States as early as 1856 when a Maryland appeals court reviewed a civil case involving the manumission of slaves.  Over the next several decades state and federal jurisdictions took up the debate as the issue of criminal insanity reached their courts of appeal.  Each court would analyze the components of M’Naghten’s Rule, and each would either adopt or modify the rule as the law governing an insanity defense in its jurisdiction.

By 1897 the appeals court of New Jersey would pronounce of M’Naghten:  “A rule of such importance, which has become so completely imbedded in the administration of the criminal law, must be considered as no longer subject to challenge.”  With a new, urban-industrial century appearing on the horizon, it seemed to that court that criminal law was now conclusive on the issue of mental responsibility, stable enough to withstand every demand upon the defense that would occur in the twentieth century.

And yet, despite the New Jersey court’s sweeping proclamation of settled law in 1897, the next 100 years would witness some of the fiercest debate, advocacy, advance and retreat in this country’s legal history, all on the issue of legal responsibility for criminal conduct committed by the mentally ill.

Eventually, in 1982 — more than a century after M’Naghten — the issues placed before the British House of Lords in 1843 would spark a similar debate in the United States following the acquittal through a successful insanity defense of our own president’s would-be assassin.

And as in England, in response to the acquittal in our country, our own legislators would convene to deliberate on the significance of the defense.  Ultimately, that attempted assassination of President Ronald Reagan would motivate our legislators not to expand our understanding and treatment of the mentally ill criminal, but rather to cap that expansion as it had been developing in the country’s federal courts.

Subsequently, over the next thirty years the country’s jails and prisons would become the country’s default residence for the chronically mentally ill.

2.7 The Best and Worst of Times