What is the Legal Definition of Insanity?

Many hear of the insanity defense in criminal cases but few actually know what the test is. Most jurisdictions employ the M’Naghten test of insanity. That is:

In California, as is in many other jurisdictions, a defendant is legally insane only if he suffers from a mental disease or defect such that at the time of the commission of the offense he was unable to understand the nature and quality of his acts, or he was unable to understand that what he was doing was wrong.

According to one local Long Beach Criminal Attorney, ten states have eliminated the “cognitive incapacity” alternative of the M’Naghten test, allowing an insanity defense only for defendants who are incapable of understanding that their actions were wrong (“moral incapacity”). Some states supplement M’Naghten with an “irresistible impulse” test. The irresistible impulse defense is available when the accused’s mind has become so impaired by disease that he is totally deprived of the mental power to control or restrain his criminal act; Under many state laws in for defendant to be considered “legally insane” the jury must find that the defendant (a) did not know the nature and quality of the act, or (b) did not know that it was wrong, or (c) was incapable of preventing himself from committing it).

A number of jurisdictions, Long Beach notwithstanding,  have adopted the Model Penal Code test for criminal insanity form, under which a person is not responsible for his criminal conduct if, at the time of such conduct, as a result of mental disease or defect, he lacked substantial capacity either to appreciate the criminality or wrongfulness of his conduct, or to conform his conduct to the requirements of law.  Special thanks to Matthew Ruff, Criminal Attorney in Long Beach.

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