Too Crazy to Be Tried – The Basics of the Insanity Defense

If someone commits a crime while being deemed legally insane, there is a high chance that they will not be found guilty. Alternatively, they may be found guilty but sentenced to a lighter punishment on the basis of their mental impairment. While not all states allow for an insanity plea, in those that do, the defendant must prove to the judge and jury that they either did not know that their actions were wrong, that they gave in to an uncontrollable impulse, that they do not know right from wrong, or any variation of the above factors.

In this article, we will explore the history and some of the current issues relating to the insanity defence, including how courts test for insanity and the difference in certain state laws and procedures.


The first recorded example of the insanity defence was recorded in a 1581 English legal treatise that stated, “If a madman or a natural fool, or a lunatic in the time of his lunacy” commits a murder, they cannot be held responsible. This treatise led to the advent of the ‘Wild Beast’ test in the 18th Century, where defendants could not be convicted if they didn’t understand the crime any better than ‘an infant, a brute or a wild beast’.

We have come a long way since then, and legal language no longer allows for terms like ‘lunatic’ or ‘wild beast’, but current legislation that allows for the insanity defence still follow the same logic as back then. The legal basis for insanity was codified into British law in the mid-19th Century with the M’Naghten Rule, which is used in a majority of U.S. states and other jurisdictions around the world today.

How Courts Currently Test for Legal Insanity

States that allow for the insanity defence in modern-day can rely on one of four different tests. Which of the four that is applicable will depend on the laws of your state. These tests are:

  • M’Naghten Rule test

Under this test, the defendant must prove that they either do not understand what they did or have an inability to distinguish right from wrong. This test operates on the assumption of a diseased mind to the extent that the sufferer is unable of understanding their own actions.

  • Durham Rule

This rule states that if the reason the crime was committed can be attributed to the defendant’s “mental disease or defect”, then the defendant is to be found not guilty by reason of insanity. This test is only used in the state of New Hampshire since all other states and jurisdictions deem the parameters for the test too broad to be used effectively.

  • The Irresistible Impulse Test

In essence, this test aims to determine whether the reason the defendant committed the crime was a result of lack of impulse control due to mental illness. If this is found to be the case, the defendant is declared not guilty by reason of insanity. Most states use this test in conjunction with the  M’Naghten Rule test, stating that when a defendant cannot understand or control their own actions, the insanity defence is applicable.

  • Model Penal Code

Under this rule, the defendant is unable to “act within the confines of the law” due to a diagnosed mental disorder. The principle is the same as the previous rules we have discussed already. This test is still used in many states, but it was criticized after it led to the acquittal of John Hinckley Jr.

State Laws

A few states don’t allow the insanity defence against criminal charges, including Idaho, Kansas, Montana and Utah. Apart from Kansas, the other three states mentioned allow “guilty but insane” verdicts, which often provide for institutionalization instead of prison. Most states that recognize legal insanity use either the M’Naghten Rule (sometimes in combination with the Irresistible Impulse Test) or the Model Penal Code. Only New Hampshire uses the Durham standard.

Do you feel you understand the Insanity Defense a bit better now? Share your views in the comments below.

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