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What Is the Insanity Defense?

By Cynthia Hsu, Esq. on August 23, 2011 | Last updated on March 21, 2019

Movies, television shows and books all seem to dramatize the insanity defense in criminal cases. But, what is the insanity defense? And, more specifically, what is the M'Naghten Rule?

If you've never heard of the M'Naghten Rule, or the Model Penal Code Rule for the insanity defense, you're probably in good company.

But, these two rules essentially make up the bulk of how states analyze the insanity defense, which is sometimes used in criminal defense cases. The insanity defense is used to argue that the defendant should not be found guilty of the crime because of their insanity, which makes them incapable to form the requisite intent to commit the crime.

To understand the insanity defense, it's necessary to understand that crimes generally have a mens rea, or mental state, element. This component requires that the perpetrator of the criminal activity commit the crime with a specific mental intent, such as knowingly committing a murder or recklessly killing someone with their car. For some crimes, a negligent mindset is also enough for a conviction.

If someone is considered "insane," they are unable to form this mental state because of their mental illness. As a result, the prosecution is not able to prove all the elements of the crime, and the defendant cannot be found guilty.

Different states have different standards on how insanity can be proven, and who has the burden of proof to prove insanity. Some states put the burden on the defendant, while others put it on the prosecution.

Some states adhere by the M'Naghten rule to determine insanity, which can be boiled down to whether or not the accused knew the difference between right and wrong when he was committing the crime.

Some other states adhere to another standard for insanity, or the Model Penal Code Rule, which deems a person insane if they do not have the mental capacity to understand that their conduct is criminal, or is unable to conform their behavior to the law.

So what is the insanity defense? Well, across all states it is something that defendants can use. But, the use and application - ranging from the M'Naghten Rule to other standards like the Model Penal Code Rule - varies depending on where you are.

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