In order for someone to be found guilty of a crime in the USA, they need to be found to have mens rea (Latin for "guilty mind") -- the intent to commit a crime ("Mens Rea," n.d.). The insanity defense exists because of the idea that some people cannot form criminal intent due to having a mental health disorder (Coric et al., 2005). Not everyone who has a mental health condition and commits a crime will be found not guilty by reason of insanity.
The insanity defense is different than the diminished capacity defense. In the diminished capacity defense, the defendent pleads guilty to a lesser charge as their mental state made it that they were not fully responsible for the crime they committed; in the insanity defense, the defendent pleads not guilty as their mental state made it that while they did commit the crime, they were not criminally responsible ("Insanity Defense", n.d.).
The insanity defense is also different than being found incompetent to stand trial. Incompetency to stand trial is not a criminal defense, but rather a reason to delay when a criminal trial will take place. Someone is found incompetent to stand trial if they don't understand the charges they are facing or if they can't effectively communicate with their attorney. If someone is found incompetent to stand trial, they will undergo treatment to restore their competency ("Insanity Defense", n.d.).
Standards to Prove the Insanity Defense
M'Naghten Test: Under the M'Naghten Test, the defendent is found not guilty by reason of insanity if they couldn't distinguish between right and wrong or did not understand what they were doing (Lewis, 2020). It got its name from a man who killed the secretary of the Prime Minister of Britain in 1843 and was found not guilty due to his belief that the government was conspiring against him (Coric et al., 2005). It is the standard test used for pleas of not guilty by reason of insanity in more than half the states of the USA and in the UK ("Insanity Defense", n.d.).
Durham Rule: Under the Durham Rule, the defendent is found not guilty by reason of insanity if the criminal act was the result of a mental condition (Lewis, 2020). It is named after a man named Monte Durham whose criminal case was used to reform the M'Naghten test. It is currently only used in New Hampshire, as it was found to be too broad and to include cases where the person did know that what they were doing was wrong ("Insanity Defense", n.d.).
Irresistable Impulse Test: With the Irresistable Impulse Test, the defendent is found not guilty by reason of insanity if they couldn't control the urge to commit a crime (Lewis, 2020). There is some worry that this test may include people who could have controlled their impulses. It also has less scientific backing than the M'Naghten test ("Insanity Defense", n.d.).
Moral Penal Code: Under the Moral Penal Code, the defendent is found not guilty by reason of insanity if they weren't able to understand the criminality of their actions or weren't able to obey the law (Lewis, 2020). This test is more flexible than the M'Naghten Test which lets the justice system acknowledge the different effects that mental health conditions can have on different people ("Insanity Defense", n.d.).
Comprehensive Crime Control Act: Under the Comprehensive Crime Control Act, the defendent can be found not guilty by reason of insanity if they weren't able to understand what they were doing or how wrong it was. The act was passed in 1984 under President Ronald Reagan ("Insanity Defense", n.d.).