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What Is The Insanity Defense?

Insanity defense working with mental health professional

The insanity defense is an affirmative defense against criminal charges. You may have heard of someone being found not guilty by reason of insanity. By using the insanity defense, a criminal defendant is telling the court that they may have committed the crime of which they are accused. But they're claiming they shouldn't be held responsible because of a severe mental health condition.

The insanity defense is unrelated to a defendant's current mental health. Rather, it applies to their mental state at the time of the crime.

This defense is often confused with diminished capacity. However, diminished capacity is not an affirmative defense that allows someone to plead not guilty in a criminal trial. Instead, the defendant may plead to a lesser crime based on their mental capacity because they were not capable of having the intent to commit the original crime with which they were charged.

The insanity defense is also different from competency to stand trial. If someone cannot understand the charges being brought against them, they cannot be tried at all.

Wondering how courts test for insanity? Unsure of state law differences or which cases have used the insanity plea? Read on for basic information and legal issues related to the insanity defense and criminal responsibility.


An English legal treatise from 1581 first records insanity as a defense to criminal charges. The treatise stated that "a madman or a natural fool, or a lunatic in the time of his lunacy" could not be culpable for killing someone.

Two centuries later, British courts came up with the "wild beast" test. A defendant who could not understand their actions any more than an "infant, brute, or wild beast" would not be culpable for their crimes. In other words, the defendant would be found not guilty by reason of insanity.

While the terms "lunatic" or "wild beast" have fallen from use, current laws that allow the insanity defense follow a similar logic. As a result, the legal basis for insanity was codified into British law in the mid-19th century.

The M'Naghten Rule (sometimes spelled as M'Naughten) was also codified. This rule was created based on the case of Daniel M'Naghten. The Scottish woodcutter killed the secretary of the British prime minister under the belief that he was conspiring against M'Naghten.

How Courts Test for Legal Insanity

Depending on the jurisdiction, courts use one or more of the following tests to establish legal insanity:

  • The "M'Naghten Rule": Here, a defendant either did not understand what they did or failed to distinguish right from wrong because of a "disease of the mind." Most jurisdictions that switched to this rule switched the burden of proving insanity from the prosecutor to the defense.
  • The "Irresistible Impulse Test": As a result of mental disease, a defendant was unable to control their impulses at the time of the offense, which led to a criminal act/criminal conduct.
  • The "Durham Rule": Regardless of clinical diagnosis, a defendant's "mental defect" resulted in a criminal act. Most state courts and federal courts abandoned this rule in 1972. As of 2023, New Hampshire courts place a strict burden of proof on the defendant when using the insanity plea. Additionally, the federal appellate judge in the Durham case decided to use the case to reform the M'Naghten rule.
  • The "Model Penal Code" test for legal insanity: Because of a diagnosed mental illness, a defendant either failed to understand the criminality of their acts or was unable to act within the confines of the law. There is a lack of mens rea or lack of a “guilty mind." A rule found in Section 4.01 of the Modal Penal Code says that a defendant lacks culpability if they did not possess a "substantial capacity either to appreciate the criminality of (their) conduct or to conform (their) conduct to the requirements of the law."

The applicable test determining criminal liability will vary from state to state depending on the state's criminal laws and individual criminal justice system.

In states such as New Jersey, the defendant may have to prove their insanity to a preponderance of the evidence. In other states and the federal court, the requirements of the law call for clear and convincing evidence. Clear and convincing evidence is a higher standard than a preponderance of evidence and, thus, harder to prove.

A minority of states place the burden of proof on the prosecution. In these states, it is up to the prosecutor to prove sanity beyond a reasonable doubt, the highest standard when it comes to burdens of proof.

State Law Discrepancies

A few states don't allow the insanity defense against criminal charges: Idaho, Kansas, Montana, and Utah. All of those states, with the exception of Kansas, allow "guilty but insane" verdicts instead of the defense of insanity. This means a defendant with a mental disorder is sent to a mental institution instead of prison.

Most states that recognize legal insanity for criminal cases use either the M'Naghten Test (sometimes in combination with the Irresistible Impulse Test) or the Model Penal Code. Only New Hampshire uses the Durham Rule.

A key fact to note is that the insane cannot be executed. This is a common law rule that the U.S. Supreme Court upheld in 1986 in Ford v. Wainwright.

Cases Involving the Insanity Defense

One famous case involving the insanity defense ended in the acquittal of John Hinckley. He was on trial for attempting to assassinate then-President Ronald Reagan. Hinckley's acquittal in 1982 was received with mixed reactions, and as a result, some states toughened their standards to make it more difficult for an acquittal to occur.

Then, in 1994, came the Lorena Bobbitt case. Faced with charges of mutilating her husband, Bobbitt had to stand trial. She was found not guilty by reason of insanity. Her acquittal led her to a maximum-security mental hospital instead of a jail cell.

Have an Attorney Answer Your Questions About an Insanity Defense

Wondering whether your case will qualify for an insanity defense? You'll need to know your state laws and whether they follow the M'Naghten Rule or the Model Penal Code. You don't have to do this alone. Learn more today by contacting a criminal defense attorney in your area. They will be happy to discuss the specific facts of your case.

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