The M'Naghten Rule

Criminal defendants who are found to be legally insane cannot be convicted of charges arising from that particular mental defect or disability. Courts use one of several legal tests to determine whether a defendant actually is legally insane, depending on the jurisdiction. They include the Model Penal Code Test; the Durham Rule; the Irresistible Impulse Test; and the M'Naghten Rule.

This article focuses on the M'Naghten Rule for testing legal insanity, often called the "right-wrong" test and used by the majority of states. See Current Application of the Insanity Defense and Status of the Insanity Defense for more information.


The M'Naghten Rule (or test) focuses on whether a criminal defendant knew the nature of the crime or understood right from wrong at the time it was committed. The defendant must meet one of the two distinct criteria. Some courts differ as to whether the "wrong" in question refers to moral or legal wrong (or both). Additionally, some states have eliminated the first part of the test in which a defendant is ruled legally insane for not fully understanding what he or she has done.

The rule, established by the English House of Lords in the mid-19th Century, states:

"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."

Examples of Insanity Using the M'Naghten Rule

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.

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.

Criticism of the 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 do 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 do not, or between temporary mental issues and lifelong conditions.

Additionally, 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.

Are You Responsible for Your Crime? Get Help From an Attorney

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

Next Steps

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