When Is a Defendant Considered Legally Insane?

What is required to prove a criminal defendant is legally insane?  Each state has its own laws and rules about its legal definition of insanity, but there are some general concepts that can be used to understand what it means to be legally insane.  It is important to note that the insanity defense is rarely used and has even been abolished in some states.

Insanity is generally considered an “excuse” to committing a crime.  An excuse defense is where, even though the defendant engaged in criminal conduct, he is excused from legal responsibility because of some condition in the defendant.

Insanity is also considered to be an “affirmative defense.”  An affirmative defense is one in which the defendant must affirmatively assert the defense and produce some evidence supporting it.  For some affirmative defenses in some states, the defendant also has the burden of proving the defense is true.

Defendants are generally presumed to be sane, so a prosecutor does not need to prove the defendant was sane unless the defendant puts his sanity into question.  Once it is put into question, either the prosecution must prove the defendant was sane beyond a reasonable doubt or the defendant must prove he was insane by a preponderance of the evidence.

In some states the defendant must enter a special plea, such as not guilty by reason of insanity. Some states have a separate sanity phase where the jury determines the defendant’s sanity after it has decided he committed the charged crime.

The focus of determining a defendant’s sanity is his mental condition at the time of the crime.  (This is one way the test for insanity is different than the test for competence to stand trial.  For a discussion of the differences between sanity and competency, check out this prior post.)

To be found insane, the defendant must show that he suffered from a mental disease or defect at the time he committed the crime.  Generally, the mental disease or defect may be permanent or temporary.

The M’Naghten Test

The defendant’s mental condition must satisfy the legal standard for insanity.  The M’Naghten test is traditionally the most commonly used  test to determine insanity.  Under the M’Naghten test, a person is considered legally insane if:

1.  at the time of the crime
2.  he suffered from a defect of reason that resulted in him either
a.  not knowing the nature and quality of the act he performed, or
b.  not knowing his act was wrong.

A defendant knows the nature of his act if he knows what act he is performing.  He knows the quality of his act if he understands the consequences that flow from his act.

For example, a woman is charged with murdering her baby by drowning.  To prove she was legally insane, she must prove that because of a defect of reason, she either did not know she was holding her baby’s head underwater, or did not know that holding the baby’s head underwater would result in him drowning.

Alternately, the defendant can show she did not know that her act was wrong.  The term “wrong” in this context may mean either legally wrong or morally wrong, depending on the state.

In the above example, if the woman knew she was holding her baby’s head underwater and understood this would cause the baby to drown, but she believed she must do so to rid him of the devil, she would assert that she did not know her act was wrong.  This would be an example of a moral wrong.

Insane Delusions

If someone suffers from delusions because of a defect of reason, they may be able to assert the insanity defense, depending on the nature of the delusion.  If the delusion was to a fact that, if true, would have justified the defendant’s acts, he could assert the insanity defense.  If, however, the delusion was to a fact that, if true, would not have justified the defendant’s acts, he may not be considered insane.

For example, the defendant was delusional and believed the victim was holding a gun when in fact e was holding a pen.  The victim pointed the pen at the defendant, who believed he was about to be shot.  The defendant pulled out a gun and shot the victim.  The defendant could assert the insanity defense because, had the victim really pointed a gun at him, he would have been justified in shooting the victim in self-defense.

However, if the defendant’s delusion was that the victim tricked him out of his money, and he shot the victim in revenge, he would not be considered legally insane because, even if his belief was true, tricking someone out of their money does not justify shooting them.

Irresistible Impulse Test

The irresistible impulse test is sometimes used in conjunction with the M’Naghten test.  Under this test, the defendant must have a defect of the mind that makes it impossible for him to control his actions.

It is not sufficient to meet this test if someone is overcome by anger, jealousy, or other strong emotions.  (Such passion may be sufficient to reduce a killing from murder to voluntary manslaughter, but it cannot be a complete defense.)

Model Penal Code Test

Another test for insanity is set out by the Model Penal Code.  Under this test a person is considered insane if, because of a mental disease or defect, the person did not have the capacity to:
1.  understand the criminality or wrongfulness of his conduct, or
2.  to conform his conduct to the law.

This is sometimes called the “policeman at the elbow test.”  In other words, if the person would have committed the act even if there had been a police officer at her elbow watching her, she is considered unable to have understood the wrongfulness of her conduct or to conform her conduct to the law.

A defendant’s sanity may be proven by either medical professionals or by lay (non-expert) witnesses.  Lay witnesses may testify about the defendant’s behavior around the time of the crime to help the jury determine whether the defendant was sane or insane at the time of the crime.


Blythe Leszkay is a successful and experienced criminal attorney, criminal law professor, and consultant to writers and filmmakers.  See About Me.  This blog is intended to answer common criminal law questions, dispel misconceptions, and explain misunderstood criminal law concepts.  It is also a place to discuss any crime or law related topics of interest.


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