What Is An Insanity Defense?

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Jeffrey Johnson is a legal writer with a focus on personal injury. He has worked on personal injury and sovereign immunity litigation in addition to experience in family, estate, and criminal law. He earned a J.D. from the University of Baltimore and has worked in legal offices and non-profits in Maryland, Texas, and North Carolina. He has also earned an MFA in screenwriting from Chapman Univer...

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Written by Jeffrey Johnson
Insurance Lawyer Jeffrey Johnson

UPDATED: Jul 16, 2021

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The insanity defense operates from the idea that people who are not mentally capable of knowing right from wrong, or who are unable to control their acts by reason of mental illness should not be held responsible for the wrongs they commit. The insanity defense acts as a not guilty defense to most crimes. Most states have at least one version of an insanity defense available for defendants to use. There are two main variations of the defense: cognitive sanity, and volitional insanity, otherwise known as the irresistible impulse defense.

Insanity Defense Variations

When using the cognitive insanity defense, the defendant concedes that he committed the crime. The most widely used basis for the defense is that the defendant was either mentally incapable of knowing the criminal quality or nature of his action; or the defendant was aware of the quality or nature of his actions, but was unable to tell right from wrong. Most crimes require the prosecution to prove beyond a reasonable doubt that the defendant had the willful intent to commit the crime. When a defendant uses the cognitive insanity defense, he is essentially alleging that he could not have had the requisite intent element, which is necessary to convict for most crimes.

Several states also allow an additional insanity defense known as the irresistible impulse defense, or volitional insanity. A defendant who uses the irresistible impulse defense also concedes that he knew his actions were criminal, but alleges that because of a mental illness or disease, he was unable to stop from committing the crime, or is unable to control his behavior. Because of the inability to control his actions, a defendant who uses the volitional insanity defense alleges that he did not commit the crime voluntarily, an element which is also necessary to convict for most crimes.

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The Insanity Defense in Practice

Since the birth of the insanity defense in the late 19th century, the acceptance of the insanity defense has alternated between rising and declining in popularity. There have been several major changes in the cognitive sanity standard over the last two centuries, and almost a complete phase out of the volitional insanity standard.

The cognitive sanity standard originated in England as the M’Naghten Rule. Under this standard, the defendant had to show that he “was laboring under such a defect of reason, from disease of mind, as not to know the nature and quality of the act he was doing; or if he did know it, that he did not know he was doing what was wrong.” This proved to be a difficult standard to meet, and during the middle of the 20th century, partially as a result in the rise of psychoanalyst theory, many states incorporated the volitional insanity test to the M’Naghten rule to allow defendants more room to prove insanity.

Other states adopted a more lenient version of the M’Naghten Rule, called the Durham Rule. While the Durham Rule was developed in New Hampshire during the late 19th century, it did not gain in popularity until the mid-1950s. The Durham Rule made it easier for defendants to show that they were insane, requiring only a showing that “his unlawful act is the product of a mental disease or defect.” However, many states found this standard difficult in application, and in 1962, the American Law Institute developed a new standard for its Model Penal Code. This new standard was based on substantial incapacity, instead of absolute incapacity, to appreciate the wrongfulness of crimes. This meant that the defendant no longer had to show that his mental illness made him completely incapable of knowing right from wrong, just that he was substantially incapable of knowing right from wrong.

The new standard was embraced by many states until 1982, when John W. Hinckley, who attempted to assassinate then-president Ronald Reagan, got acquitted by reason of insanity. As a result of the public outrage at the acquittal, many states reformed the insanity defense standard again, making it more difficult for a defendant to prove he is not guilty by reason of insanity.

Many states reverted back to the M’Naghten test, which is more difficult for the defendant to prove. Other states extended minimum terms for defendants who are found not guilty by reason of insanity in mental institutions. A few states have discarded the insanity defense all together.

Almost all of the states that have kept the either the American Law Institute or the M’Naughten insanity standards have also toughened the standards. In many states, instead of the burden on the prosecution to show that the defendant is sane, the burden lies on the defense to show that the defendant is insane. This is different than other defenses used in criminal trials, where the burden is generally on the prosecution to show why the defendant’s defense is false. Once the defendant meets his burden, the insanity defense is considered at either the guilt or sentencing proceedings. In either scenario, the judge will generally instruct the jury that they are able to find the defendant not guilty by reason of insanity if they find that the weight of the evidence is in fact sufficient to show insanity.

Controversy of the Insanity Defense

Critics of the insanity defense argue that allowing this defense gives guilty defendants an opportunity to fake insanity, and escape liability for horrific crimes like murder. There have been several high-profile murder cases in which the insanity defense was successful, which have caused public outcry. In fact, approximately 70% of defendants who invoke the insanity defenses are for crimes other than murder.

Further, studies show that the insanity defense is rarely used; it is estimated that about 1% of defendants nationwide use the defense at all. When it is used, it is only successful about 20% of the time. The insanity defense is used most successfully in plea bargains with the prosecution.

Finally, defendants who use the insanity defense successfully are generally not just let loose on the streets. Defendants found not guilty by reason of insanity are often committed to a mental health facility for an indefinite duration of time, usually until the individual is no longer considered to be a threat. This in itself can amount to a prison sentence, as defendants have, on average, been committed to mental treatment centers for double the time they would have been sent to prison.

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