The so-called “insanity defense” has been a staple of crime dramas on television, film, and in real life for decades. The sensationalized version is something like this: A person commits a terrible crime, such as murder. In court, they do not deny that they killed the person. Instead, their attorney argues that the defendant was simply too insane to realize what they were doing. They should not be sent to prison for committing a crime they were unable to avoid or that they did not even know they were committing. A TV show would then look to the emotional fallout of the victims and the community when an apparent crazed murderer gets away with the crime.

In the real world, the insanity defense is an actual, technical legal defense. It is a traditional affirmative defense that goes back as far as English common law. Employing the defense involves arguing that a criminal defendant suffered from a mental illness that prevented them from knowing that their actions were wrong. The insane defendant did not, and indeed could not, know that assaulting a person, taking something that did not belong to them, or stabbing a victim was wrong, and certainly not that it was against the law. The law does not like to send people to prison who had no control over their actions or who had no way of knowing that they were behaving contrary to societal norms.

In practice, if someone commits a very serious crime such as murder and relies on the insanity defense, they would not simply be set free after the trial. Instead, if they pose a danger to other people, they would likely be committed to a mental institution. They may not go to prison, but they will not be left free in the world to pose a danger to other people. Until recently, every state permitted some form of the insanity defense as an affirmative defense to criminal charges.

Can a State Legally Get Rid of the Insanity Defense?

In light of laws recently passed in a small handful of states, the Supreme Court is now tasked with determining whether the insanity defense is a constitutional right. Five states, including Alaska, Idaho, Kansas, Montana, and Utah, have recently passed laws abolishing the insanity defense. In Kahler v. Kansas, the Supreme Court will determine whether the Eighth and Fourteenth Amendments prohibit those states from removing that traditional defense.

Kahler involves a man who was arrested and charged with four counts of first-degree murder after killing four family members: his ex-wife, his wife’s grandmother, and his two teenaged daughters. The defendant provided testimony from a forensic psychiatrist who stated that Kahler was suffering from severe depression at the time of the crime. The expert testified that the defendant’s “capacity to manage his own behavior had been severely degraded so that he couldn't refrain from doing what he did. He relied on the so-called M’Naghten rule, which holds that “the defendant is to be held not criminally responsible (1) where he does not know the nature and quality of his act, or, in the alternative, (2) where he does not know right from wrong with respect to that act.”

Kansas abolished the M’Naghten rule in 1996, instead adopting an approach to criminal intent (mens rea) that “allows evidence of mental disease or defect as it bears on the mental element of a crime,” but which “abandons lack of ability to know right from wrong as a defense." Kahler was convicted and appealed to the Kansas Supreme Court, claiming his Due Process rights were violated when Kansas chose to ignore the “right and wrong” insanity defense.

The case is now before the U.S. Supreme Court. The parties argued before the Court in Kahler v. Kansas in October 2019. The Court is expected to issue its decision sometime this term, before the summer of 2020. The decision could have a lasting, significant impact on the development of criminal law in the U.S.