A criminal defendant who is found to be legally insane at the time he or she committed the crime at issue may be found not guilty by the reason of insanity. This can be crucial in a criminal case because even if the defendant committed the crime, since there was no general or specific intent for the crime to be committed, they would not be liable for the cause. In some cases, if the defendant is found guilty, the insanity defense can be a mitigating factor in the sentencing phase due to the mental impairment, same with the defense of diminished capacity.

If it is established that the defendant was legally insane at the time the crime took place, he will be completely excused. Upon a finding a mental insanity, the court will generally order the defendant to be placed in a mental hospital.

How Can a Criminal Defendant Establish an Insanity Defense?

In order to establish an insanity defense, the defendant must satisfy one of the following tests, depending upon the jurisdiction where the crime was committed:

  • M’Naghten Test: A criminal defendant either 1) did not understand his actions’ nature and quality, or 2) lacked knowledge of the wrongfulness of his actions.
  • Irresistible Impulse Test: A criminal defendant couldn’t control his actions. In another words, this test emphasizes defendant’s volitional incapacity.
  • American Law Institute Test: A criminal defendant lacked substantial capacity to understand wrongfulness of his actions or control them. Here, a total lack of understanding or control is not required. The defendant must have been substantially unable to understand or control his actions.
  • Durham Test: A criminal defendant would not have committed the crime "but for" his mental disease, of which his crime was a product. With the exception of New Hampshire, this test is not in use today.
  • Federal Test: As a result of several mental disease, criminal defendant couldn’t understand wrongfulness of his actions or couldn’t control them. This test is similar to the one used by American Law Institute. However, the defendant has to prove "severity" of his mental illness and do so by clear and convincing evidence, a higher standard then preponderance of evidence.

Note that in most jurisdictions, a criminal defendant has the initial burden of producing evidence of legal insanity, which must be proven by a preponderance of the evidence. In some states, the prosecution must prove beyond a reasonable doubt that the defendant was sane when he committed the crime.

What is a Diminished Capacity and How Does it Differ from Insanity?

Some states recognize a defense of "diminished capacity," which is basically a mental defect that falls short of insanity. Diminished capacity is not a complete defense. It usually serves to mitigate a criminal defendant’s conviction to a less serious charge. A criminal defendant may claim diminished capacity in connection with a mental defect as well as intoxication. In jurisdictions where diminished capacity is recognized, it often helps establish a defense for specific intent crimes. 

Does Incompetency to Stand Trial Serve as a Defense?

The question of a criminal defendant’s competency to stand trial is separate from the issue of his insanity at the time the crime was committed. If a defendant lacks the capacity to assist his lawyer or understand the proceedings brought against him, due process prevents his trial, conviction, and sentencing. However, if a criminal defendant regains competence, proceedings may resume. Finally, while the jury must decide the issue of the defendant’s insanity defense, a judge makes the determination regarding his competency to stand trial.

Who Has the Burden of Proof to Establish Whether the Defendant Was Insane?

The question of who has the burden of proof to an insanity defense has been a major controversy. Before the verdict was decided in the Hinckley case, the majority of states had the burden of proof to establish whether the defendant was insane at the time of the crime rest with the state and prosecution. After the Hinckley verdict, a majority of states changed this opinion and required the defense to prove that the defendant was in fact insane at the time of the crime. In states that the burden is on he defense to prove insanity and mental health issues, the defense is required to show either by clear and convincing evidence or by a preponderance of evidence that the defendant was legally insane.

In states which the state and prosecutor have the burden of proof, the prosecution is required to prove that the defendant was not insane beyond a reasonable doubt.

How to Enter a “Not Guilty by Reason of Insanity” Plea:

A defendant who pleads the insanity defense is required to prove that he or she was insane at time of the crime by a "preponderance of evidence". Preponderance of evidence means that he or she has to show that is is more likely than not that he or she was insane. There are two ways to purse an insanity defense:

1. Plead both "not guilty" and "not guilty by a reason of insanity, OR

2. Admit that you committed the crime, but plead "not guilty by reason of insanity".

If the defendant is found guilty and this defense was entered, the case will proceed to a sanity hearing which is called a "sanity trial". This hearing will involve the same jury who made the original guilty verdict.

During the sanity hearing, the defendant has the burden to prove the defense by presenting things like:

  • Mental health documents showing a history of mental illness
  • Presentation of expert witnesses stating that the defendant was insane at the time of the offense
  • Presentation of hospital records showing a history of mental history

To prove insanity in court, the criminal defense attorneys must prove any type of evidence that their client’s mental illness prevented the defendant to understand the wrongness of his actions or the nature of his or her conduct at the time of the offense. This standard is generally more strict than merely showing that the defendant could not tell between right and wrong.

Do I Need an Attorney’s Help?

If you are faced with criminal charges, a qualified criminal defense attorney will help you understand the defenses available to your case. An attorney will help you develop a strategy for your defense and will represent you during trial.