Skip to main content

Verified by Psychology Today

How to Plead Insanity

"Insanity" is a legal term, not a medical term.

Although the insanity defense is extremely popular in the media, books, and pop culture, law students are taught that the insanity defense is rarely used, and even more rarely successful. While this lesson is true, it does not change the fact that sometimes the insanity defense is successful, and for whatever reason, it leaves many people feeling uneasy.

Take recent news, for example. Two months ago, a Texas mother who admitted to killing her two special-needs children because she wanted “normal kids” was found not guilty by reason of insanity.[1] Last month, a California man who shot his estranged wife's friend and beat her severely was found not guilty by reason of insanity.[2] Just last week, an Iowa teenager who killed his five-year-old foster brother with a brick was found not guilty by reason of insanity.[3]

These cases force us to face the reality of the insanity defense, and raise many questions: what does it mean to be "not guilty by reason of insanity"? Do you have to be a raving lunatic or just mentally disturbed? Do you have to be diagnosed? Do personality disorders count? How is a jury with no medical experience qualified to determine if someone is insane? As it turns out, everyone has heard of the insanity defense, but not very many people understand it.

The key to dispelling the confusion is this: here, "insanity" is a legal term and not a psychiatric or medical term. Therefore, it will have a legal definition that sometimes overlaps with medical diagnoses of mental illnesses, but is by no means interchangeable.

The legal definitions of insanity vary by state. In other words, someone who is found legally insane in New Hampshire might not be found legally insane in New York, even if the juries agreed on all of the facts. States generally apply one of three rules, or at least a variation of one of three rules, to define "insanity."

Under the M'Naghten rule, a criminal defendant is not guilty by reason of insanity if, at the time of the alleged criminal act, the defendant was so deranged that she did not know the nature or quality of her actions or, if she knew the nature and quality of her actions, she was so deranged that she did not know that what she was doing was wrong. The M'Naghten Rule is the oldest rule, originating from England in the 1800s, but still applies in over 20 states today, including California and Florida.

Under the Durham Rule, a criminal defendant is not guilty by reason of insanity if the crime was the product of his mental illness (i.e., crime would not have been committed but for the disease). This rule is much broader than the M'Naghten rule because it would excuse a defendant who understood the nature of his criminal action or appreciated that it was wrong, but did it anyway because of a mental illness. New Hampshire is the only state that follows the Durham Rule.

Under the Model Penal Code Rule, a criminal defendant is not guilty by reason of insanity "if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law." This Rule was written by the American Law Institute as a compromise between the narrow M'Naghten Rule and the broad Durham Rule. The Model Penal Code Rule has been adopted by many states, including Massachusetts, New York, Michigan, and Hawaii.

Four states do not recognize the insanity defense at all: Montana, Idaho, Kansas and Utah.

In sum, even though "insanity" is a legal concept, and not a medical concept, there is no consensus on what it means in the legal community.

Say a man who is mentally ill hears voices telling him that if he does not strangle his wife, he will be abducted by aliens. He understands that murder is wrong, but fears the aliens so much that he commits the crime anyway. Under the M'Naghten test, his insanity defense would most likely fail because he knew the nature of his act and that it was wrong. Under the Durham Rule, his insanity defense would most likely succeed because he would not have killed his wife if he had not been mentally ill. Under the Model Penal Code Rule, the answer is much less certain. Although he did not lack the substantial capacity to appreciate the criminality of his conduct, it is unclear whether he lacked the substantial capacity to conform his conduct to the requirements of the law. In a case like this, it would be ultimately up to a jury—not a psychiatrist or doctor—to determine whether the defendant was truly "insane."


Disclaimer: This article is for informational purposes only and not for the purpose of providing legal advice. You should contact your attorney to obtain advice with respect to any particular issue or problem. Use of and access to this blog or any of the e-mail links contained within the site do not create an attorney-client relationship between the author and the user or browser. The opinions expressed at or through this site are the opinions of the individual author and may not reflect the opinions of any law firm or Psychology Today.

News Sources: