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The Insanity Plea: When and Why It’s Used

Brian J Smith May 28, 2018

“Insanity” seems like a notion that you can’t define, but you know it when you see it. If you think it’s hard, it’s even more difficult to nail down as a legal term. That’s why not guilty by reason of insanity or mental disorder as a legal defense is so rarely used successfully in the United States. But some defendants do qualify, so let’s dig a little deeper.

Varying Tests

The legal definition, and even the ability to use it as a defense in a criminal trial, varies from state to state. In fact, four states don’t accept the defense at all: Idaho, Kansas, Montana and Utah.

The rest accept insanity as a legal defense in a few different ways, or variations thereof.

The M’Naughten (sometimes spelled M’Naghten) Rule

This test goes back to England in the 1840s. It basically means that defendants are so deranged that they don’t even know that what they did was wrong. Only the state of New Hampshire follows this test today.

The Durham Rule

This states that the defendant’s mental illness was so powerful that it overruled defendants’ ability to refrain from criminal action even if they knew that what they were doing was wrong.

Model Penal Code Rule

This test is sort of halfway between the very narrow M’Naughten Rule and the more liberal Durham Rule. It describes a defendant as lacking the capacity to appreciate the criminality of the action or to follow the law, due to mental illness.

Jury’s Decision

If these various legal definitions of criminal insanity seem a bit vague, it’s because of the difficulty in looking into a human mind. We can determine without a doubt that an individual has lung cancer or a fractured leg, but a “broken” mind is much harder to diagnose in a courtroom.

Mental state will usually be debated by dueling psychiatric experts and the final determination left to jurors with no expertise in the field whatsoever.


Defendants who’d like their trial attorney to present an insanity defense should be aware that even success could result in a form of incarceration that’s little different than prison. You could be sent to a mental hospital for an indeterminate period, until you’re declared well again. In some cases, this could result in a loss of freedom for even longer than a prison term would bring.

However, there are times when an insanity plea is simply the right thing to do, and when it is we will pursue it vigorously.