Background and History of the Insanity Defense

The insanity defense is perhaps the most controversial of all criminal defense strategies. It's also one of the least used and least successful defenses. Despite this, several successful uses of the defense have caused public outrage in the United States and the United Kingdom. In some cases, it led to reforms of the defense.


The insanity defense has roots in ancient Rome and Greece and has evolved over time. Today, most states use one of two insanity defenses, described below.

This article describes the basis for the insanity defense. It then provides a brief history of the defense's evolution from ancient to modern times. The article concludes with several reforms proposed by critics of the insanity defense.

The Basis for the Insanity Defense

The insanity defense asserts that a criminal defendant is not criminally liable for their illegal acts due to their insanity. Insanity, in this context, refers to a specific mental illness or mental disorder.

A defendant asserting an insanity defense doesn't argue whether they committed the alleged crime. Instead, the defense is whether the law should hold them liable for their criminal behavior since they did not have the mental state to commit the crime.

In criminal law, the prosecution has the burden of proof in proving a criminal is guilty beyond a reasonable doubt. Most crimes require the prosecution to prove two things:

  • Mens rea: The alleged criminal intended to commit an act and bring about a desired result
  • Actus rea: The act which the criminal intended occurred and was criminal

For example, suppose the government alleges Nick murdered someone by stabbing them with a knife. The government must prove beyond a reasonable doubt that Nick intended to stab the victim to cause them harm and that stabbing someone is a crime.

The theory behind the insanity defense is that a person who is insane lacks the intent required to perform a criminal act. For instance, if Nick's mental illness made him unable to know the act was wrong, or he could not control their actions even though he knew they were wrong, he may successfully assert the defense. If he successfully asserts the defense, the jury may find him not guilty by reason of insanity.

This theory is controversial because insanity itself is difficult to define, and the circumstances in which a defendant may use an insanity defense to excuse criminal responsibility are challenging to characterize.

History of the Insanity Defense

The insanity defense has a long history in criminal law. Courts in ancient Rome and Greece provided some defenses for people whose mental disease led them to commit crimes.

Initially, courts didn't consider the insanity defense an argument that could acquit the defendant. Instead, it was a way for a defendant to receive a pardon or mitigate a sentence.

This section summarizes the development of the insanity defense.

The 'Wild Beast' Test (1256)

The history of the modern insanity defense stretches back to at least the 1200s. The so-called “wild beast" test was one of the first tests for insanity. It originated in Judge Henry de Bracton's treatise, which was written in 1256.

An English case in 1724 described an iteration of the wild beast test. There, the court set out a person is insane if they were “totally deprived of his understanding or memory, and doth not know what he is doing, no more than an infant, a brute, or wild beast."

The wild beast test gave wide discretion to the trier of fact regarding whether a defendant possessed the required state of mind to commit a crime due to their alleged diminished capacity. But lacking a better option, courts used the wild beast test for centuries.

The M'Naghten Test (1843)

The idea that insanity could bar a defendant's conviction arose in the early nineteenth century. A founding member of the American Psychological Association, Isaac Ray, wrote “A Treatise on the Medical Jurisprudence of Insanity" in 1838. Soon after, a criminal defendant's attorney relied on the treatise in the seminal decision called the M'Naghten case.

Daniel M'Naghten had a mental disease causing him to experience paranoid delusions. He believed the Catholic church and British government sent spies after him and conspired to kill him. He planned to kill the Tory Prime Minister, Sir Robert Peel. Ultimately, he killed Peel's secretary, Edward Drummond, in 1843.

At M'Naghten's criminal trial, his attorney relied heavily on Ray's treatise. The treatise generally stated that the determining factor in an insanity defense is whether the defendant could tell right from wrong at the time of the crime.

The jury returned a verdict of insanity and acquitted M'Naghten. The court institutionalized M'Naghten for the rest of his life.

The verdict upset the ruler of Britain, Queen Victoria. She requested the House of Lords discuss and clarify the insanity defense. The House of Lords debated the issue and created the so-called “M'Naghten rule." The rule states that, for a defendant to assert and prevail on an insanity defense, they must show the following:

  1. They had a “defect of reason" from a “disease of the mind" (i.e., a mental illness) at the time of their criminal act; and
  2. Due to the disease, they did not understand the nature or quality of their criminal act; or
  3. If they understood the act's nature, they could not appreciate whether the act was right or wrong

American jurisdictions and the criminal justice system adopted the M'Naghten test in criminal cases. It set a new standard for legal insanity in United States jurisprudence.

New Hampshire (1871)

A New Hampshire court created a new test to determine insanity in 1871. In doing so, it rejected the M'Naghten test. The New Hampshire court ruled that a defendant is not liable for their criminal act if the act resulted from their mental illness.

The New Hampshire rule treats insanity as a question of fact rather than a question of law. As one professor notes, the test acknowledges that no universal test exists to define and determine insanity. It leaves to the jury the question of whether a defendant did not act on their own volition due to a mental defect or whether the act was simply a deviation from their normal personality.

The Irresistible Impulse Test (1887)

The Alabama Supreme Court rejected the M'Naghten test in 1887. There, the court created the Irresistible Impulse test. The idea behind the test is that, due to their mental state, the defendant could not control an impulse to commit a criminal act, even though they knew it was wrong.

The test requires the defendant to show the following:

  1. They have a mental illness; and
  2. The mental illness caused an inability to control their actions or conform their actions to the law

Several states adopted and continue to use the Irresistible Impulse test. Others use it to supplement the M'Naghten test.

Critics argue the test prevents people with mental illnesses from asserting it unless their mental illness caused a total inability to control their actions. It's also difficult to determine whether the defendant truly had an irresistible impulse rather than an impulse they simply did not resist.

The Durham Rule (1954)

The Durham rule states that a criminal defendant is not criminally liable for an act resulting from a mental illness they had at the time of the crime. It comes from the Washington D.C. federal court case Durham v. United States.

In Durham, the defendant had a history of mental illness and a lengthy criminal record. Courts had institutionalized him three times. The mental health institution released him three times, and he committed criminal acts following each release. This particular case involved an alleged criminal act he committed two months after his third release.

The Durham court created a new insanity defense rule in 1954. Criticizing the M'Naghten rule and Irresistible Impulse test, the court noted that an insanity test focusing on “right or wrong" was “obsolete." The court cited several organizations and mental experts stating the same. The court wrote, “The science of psychiatry now recognizes that a man is an integrated personality and that reason, which is only one element in that personality, is not the sole determinant of his conduct." Due to this, the court concluded that a broader test was appropriate, and it created the Durham rule.

The Durham rule, also known as the product test, states that “an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect." The Durham rule emphasizes expert testimony regarding whether a defendant's alleged acts resulted from a mental impairment. The rule, however, did not gain acceptance from other federal courts. The U.S. Court of Appeals for the District of Columbia Circuit abandoned it in 1972.

Model Penal Code Test (1962)

The American Law Institute's (ALI) Model Penal Code (MPC) created an insanity defense in 1962. The MPC test states that a criminal defendant is not criminally liable if their conduct resulted from a mental disease or defect and:

  1. The defendant lacked the substantial capacity to appreciate the act was criminal; or
  2. The defendant could not conform their acts to the requirements of the law

Many jurisdictions adopted the MPC test. But it faced criticism following the trial of John Hinckley, Jr. in 1982.

Hinckley attempted to assassinate President Ronald Reagan in 1981. During his trial, evidence indicated he tried to kill Reagan to impress actress Jodie Foster. He drew inspiration from the movie “Taxi Driver," in which the male protagonist attempts to save Jodie Foster's character and kill a United States senator. Hinckley wrote a letter to Foster before the attempted assassination, indicating he would not have tried to kill Reagan if he could win Foster's heart.

There was no question that Hinckley attempted the assassination. The true question to the jury was whether Hinckley was criminally liable for the attack or if he could assert an insanity defense. Expert witnesses testified, some for several days, as to Hinckley's mental state.

Hinckley's defense attorneys rested their case after a full viewing of “Taxi Driver" in court. The defense introduced the movie as evidence of Hinckley's insanity. The jurors reportedly saw Hinckley take in the film with great interest. He reportedly covered his eyes during a scene in the movie involving Foster's character embracing another character.

The jury found Hinckley not guilty by reason of insanity in 1982. The court institutionalized him. In 2016, the mental health institution released him with several conditions. In 2022, a federal judge released him free of any conditions.

Hinckley's acquittal caused public outrage. Soon after, Congress passed the Insanity Defense Reform Act of 1984 (IDRA). The IDRA shifted the burden of proof in an insanity plea from the prosecution to the defendant. Following the Hinckley decision, some states, like Idaho, Montana, and Utah, abolished the insanity defense.

The Insanity Defense Endgame

Most states use the M'Naghten test or the MPC test, but it varies from state to state. Utah, for example, abolished the insanity defense, but it allows for a “guilty but mentally ill" (GBMI) conviction. In a GBMI conviction, a court considers the defendant liable for their criminal acts, but they may receive mental health treatment evaluation from mental health professionals. Instead of a prison sentence, a Utah court may hospitalize the person as appropriate.

Defendants rarely raise an insanity defense. Of those that assert it, very few prevail. According to research, less than 1% of all criminal defendants raise the defense. Of those defendants, less than 30% prevail.

Given its low success rate and general public hostility to insanity pleas, critics have suggested reforms to its current form.

One possible reform involves changing the defense altogether. For example, instead of a verdict of “not guilty by reason of insanity," the GBMI verdict could replace it. Another alternative is “not responsible by reason of insanity." Supporters of these alternatives argue that the term “insane" is outdated and carries a stigma. The proposed alternatives, they say, are more accurate descriptions of the verdict and may change public perception of the insanity defense.

Another criticism involves the fact that courts frequently commit those acquitted of a crime to a mental institution. These institutions may have conditions as bad as, or worse than, a prison. Also, those held in mental institutions generally do not know if or when the institution will release them. According to Dr. Charles Patrick Ewing, author of “Insanity: Murder, Madness, and the Law," most convicted people sent to mental institutions will spend more time in a mental institution than they would have if the court sentenced them to prison.

A suggested reform for the mental institution issue is to incorporate more mental health resources into prisons, jails, or detention centers. Supporters of this reform argue it would better equip such facilities to help prisoners with mental illness and keep them safe.

Get Help With Your Criminal Defense From a Legal Professional

If you believe you have a viable insanity defense, contact a local criminal defense lawyer today. An experienced criminal defense attorney can provide you with information regarding the following:

If you're facing criminal charges, don't delay in contacting a criminal defense attorney near you. They can provide critical defense strategies for your specific legal issue.

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