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The Four Tests Used for Determining Legal Insanity

The Four Tests Used for Determining Legal Insanity

The insanity defense often stirs up quite a bit of debate, especially among members of the public who may not be fully aware of how courts determine whether or not a person is legally insane. It should not be surprising that “not guilty by reason of insanity” provokes such strong feelings. After all, in such cases, the accused has admitted to committing the crime but is nonetheless found not guilty because he or she lacked the mental competence to understand that what he or she was doing was wrong. Courts throughout the United States typically rely on a number of tests for determining whether the accused was legally insane when an offense was committed. Here are the four most important such tests used for determining legal insanity.

The M’Naghten Rule

The M’Naghten rule, named for Englishman Daniel M’Naghten, who was found not guilty by reason of insanity in 1843 for murdering the Prime Minister’s secretary, is used by a majority of states and was fundamental in establishing the insanity defense. With the M’Naghten rule, a defendant is deemed to be legally insane if he or she was unaware of what he or she was doing when the offense was committed or, even if the defendant knew what he or she was doing, that defendant was incapable of understanding that what they were doing was wrong.

The “Irresistible Impulse” Test

In some cases, however, a defendant may know that his or her actions were wrong, but committed them because of an “irresistible impulse.” The “Irresistible Impulse” test is used by a number of states in combination with the M’Naghten rule. With the “Irresistible Impulse” test, the focus is on volition. Essentially, the test allows for a defendant to be found not guilty by reason of insanity if his or her mental illness meant that, although recognizing the wrongness of the offense, he or she was compelled to commit the offense anyway.

Tests Used for Determining Legal Insanity

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The Durham Rule

Today used only in New Hampshire, the Durham rule places a great deal of emphasis on scientific psychological evaluations and evidence. In most cases, juries follow the diagnoses made by trained professionals in determining whether the accused is guilty. This test has largely fallen out of favor, however, since it takes much of the decision-making abilities out of the jury and places it in the hands of psychologists (who, sometimes, may even disagree among themselves about a defendant’s insanity).

The Model Penal Code

The Model Penal Code is an updated definition of the insanity defense and addresses some of the weaknesses of the above tests. The Model Penal Code tends to be much broader than the relatively rigid M’Naghten rule, but also incorporates the centrality of the defendant’s volition that is addressed by the “Irresistible Impulse” test. As such, it is usually used by states that do not use the M’Naghten rule. The Model Penal Code also prohibits psychopaths and sociopaths from using the insanity defense.

The four tests described above are the most important ones for helping a court determine a defendant’s claim that he or she was legally insane when an offense was committed. While the insanity defense has long been controversial, these tests help ensure that criminal justice remains fair even in cases involving severe mental illness.


This article contains general legal information but does not constitute professional legal advice for your particular situation. The Law Dictionary is not a law firm, and this page does not create an attorney-client or legal adviser relationship. If you have specific questions, please consult a qualified attorney licensed in your jurisdiction.

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