Mental Disease or Defect


Fundamentals of Criminal Law

Adam J. McKee, Ph.D.

Jack Brown, Ph.D.


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Although widely unpopular, most states and the federal government recognize some sort of insanity defense (e.g., 18 U.S.C. § 17).  

The term insanity comes from the law; psychology and medicine do not use it. The everyday use of the term can be misleading. If a person acts abnormally, they tend to be considered by many as “crazy” or “insane.” At law, merely having a mental disease or mental defect is not adequate to mitigate guilt. It must be remembered that Jeffrey Dahmer was determined to be legally sane, even though everyone who knows the details of his horrible acts knows that he was seriously mentally ill. To use insanity as a legal excuse, the defendant has to show that he or she lacked the capacity to understand that the act was wrong, or the capacity to understand the nature of the act. Some jurisdictions have a not guilty by reason of insanity plea.

The logic of the insanity defense goes back to the idea of mens rea and culpability. We as a society usually only want to punish those people who knew what they were doing was wrong. Most people believe that it is morally wrong to punish someone for an unavoidable accident. Likewise, society does not punish very young children for acts that would be crimes if an adult did them. The logic is that they do not have the maturity and wisdom to foresee and understand the nature of the consequences of the act. Put in oversimplified terms, if a person is so crazy that they do not understand that what they are doing is wrong, it is morally wrong to punish them for it.

Over the years, different courts in different jurisdictions have devised different tests to determine systematically if a criminal defendant is legally insane. One of the oldest and most enduring tests is the M’Naghten rule, handed down by the English court in 1843. The basis of the M’Naghten test is the inability to distinguish right from wrong. The Alabama Supreme Court, in the case of Parsons v. State (1887), first adopted the Irresistible Impulse Test. The basic idea is that some people, under the duress of a mental illness, cannot control their actions despite understanding that the action is wrong.

Today, all of the federal courts and the majority of state courts use the substantial capacity test developed within the Model Penal Code. According to this test, a person is not culpable for a criminal act “if at the time of the crime as a result of mental disease or defect the defendant lacked the capacity to appreciate the wrongfulness of his or her conduct or to conform the conduct to the requirements of the law.” In other words, this test contains the awareness of wrongdoing standard of M’Naghten as well as the involuntary compulsion standard of the irresistible impulse test.

It is a Hollywood myth that many violent criminals escape justice with the insanity defense. In fact, the insanity defense is seldom attempted by criminal defendants and is very seldom successful when it is used. Of those who do successfully use it, most of them spend more time in mental institutions than they would have spent in prison had they been convicted. The insanity defense is certainly no “get out of jail free card.”

Modification History

File Created:  07/17/2018

Last Modified:  07/17/2018

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This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.

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