The M'Naghten Rule

Created by FindLaw's team of legal writers and editors.

There are a variety of criminal defense strategies that an attorney may utilize when defending a client in a criminal case. But, despite how it's portrayed in the entertainment industry, the insanity defense isn't easy to use. In order to be not guilty by reason of insanity, a criminal defendant must meet the jurisdiction's definition of legal insanity.

Courts will determine legal insanity by applying one of the following tests/rules:

It's not up to the court to decide which test to apply; instead, it's determined by the laws of a particular state. This article focuses on the M'Naghten Rule for testing legal insanity, which is used by a majority of the states.

Overview of the M'Naghten Rule

The M'Naghten Rule (or test) was established by the English House of Lords in the mid-19th Century and states that:

"Every man is to be presumed to be sane, and ... that to establish a defense on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was laboring under such a defect of reason, from disease of mind, and 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."

Basically, this test focuses on whether a criminal defendant knew the nature of the crime or understood right from wrong at the time it was committed. Thus, in order to be declared legally insane under this test, a defendant must meet one of these two distinct criteria.

In applying this test, courts may differ as to whether the "wrong" in question refers to moral or legal wrong (or both). Additionally, some states have eliminated the criteria which defines a defendant as legally insane for not fully understanding what they've done.

Examples of Insanity Using the M'Naghten Rule

The best way to get a better understanding of this type of legal insanity is to see a few examples of how the rule may be applied to certain scenarios.

Example 1: A man murdered his wife and daughter, and then waited calmly for the police to arrive. Three mental health experts testified that he was too psychologically ill to understand that his criminal acts were wrong. He was found not guilty by reason of insanity and sentenced to 10 years in a mental health facility.

Example 2: A woman with severe schizophrenia is charged with assault and battery after attacking her next door neighbor with a shovel. She claims the neighbor was actually a demon who was trying to harvest her soul. She was found not guilty by reason of insanity after the court determined that she failed to understand the nature of her actions.

The above examples show the two different ways that a defendant may be declared legally insane under this rule.

Criticism of the M'Naghten Rule

This particular test for legal insanity has been challenged for a number of reasons. Some have argued that defendants meeting the legal definition of insanity don't always meet the medical criteria for insanity, but are sentenced to mandatory medical care anyway.

Another criticism is that it fails to distinguish between defendants posing a public danger and those who don't, or between temporary mental issues and lifelong conditions. Finally, some have argued that this rule makes it too easy for a defendant with a severe mental disorder to escape responsibility for any crimes, regardless of how big a role the disorder played in the incident.

Learn More About the M'Naghten Rule: Speak to an Attorney

As you can see, the insanity defense has very specific criteria in order to qualify and can carry lifelong consequences. If you're facing criminal charges and want to know more about the standards of an insanity defense, your first step should be to speak with a knowledgeable attorney. Find out more and speak with a local criminal defense attorney today.

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