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A Brief History of the Insanity Defense

How the M’Naghten Rule came to exist.

The insanity defense as a legal concept was born in England, in 1843. A man named Daniel M’Naghten attempted to assassinate the British Prime Minister who he believed was conspiring against him. Due to his psychosis, the court acquitted him and thus established the Mr. M’Naghten Rule. It requires that a defendant is to be found not guilty of an offense if, at the time it occurred, his mental disorder was so grave as to (1) interfere with his ability to know or understand the nature or quality of his criminal behavior, and (2) to have compromised the defendant’s ability to know or understand the legal or moral wrongfulness of his behavior. This two-pronged rule became the legal standard for an insanity defense in the United States as well.

Fast forward several decades when an American jury had to consider the fate of Charles Guiteau after he successfully assassinated President James Garfield on July 2, 1881. At his trial, Mr. Guiteau claimed to be an agent of God when he shot the President. His defense team argued that their client believed he was responding to a deific decree and following God’s command when he assassinated the president. He certainly gave the jurors ample evidence of his bizarreness. Interrupting and constantly insulting his defense team in court, he sang and recited poems and he solicited legal advice from courtroom strangers.

Nonetheless, the jury found the defendant guilty of murder. They did not believe he met the M’Naghten standard of insanity. Soon afterward, the courts in some jurisdictions established Deific Degree Doctrine as another prong to the insanity defense which, when called for, may be formally considered during insanity hearings. Though not recognized in California as a prong of the insanity defense statute per se, a delusional belief about a divine command still requires an analysis of its implication regarding a defendant’s comprehension of moral wrongfulness. Believing one has received a deific command can obscure an understanding of moral righteousness, as God is considered the ultimate arbiter of moral conduct, beyond man-made laws. The hard question that needs to be resolved and opined upon by jurors is whether the defendant actually believed he (or she) was commanded by God to break the law.

When researchers survey people about the insanity defense, most express a great deal of dissatisfaction about it, and with mental health defenses in general. The public generally feels it is used by criminals to avoid their "just desserts." This isn’t surprising, given the media’s attention to cases like the Son of Sam (David Berkowitz) case I profiled in the preceding blog. Berkowitz first claimed he killed by order of a demon, transmitted to him via his neighbor’s dog. He later admitted it was a sham excuse. It is notorious cases like this that retail as prototypical of the insanity defense claims. The facts tell a different story. The insanity defense is employed at an extremely low rate, less than 1%. As a defense, it’s rejected by the trier of fact 75% of the time.[i] And those 25 percent that are found insane usually have an unequivocal history of severe mental illnesses that were manifestly active at the time of the crime.

Conducting an insanity evaluation and rendering an opinion on a defendant’s state of mind at the time of a crime is among the most complex tasks faced by a forensic psychologist. It’s a weighty responsibility, particularly when the crime was heinous, the victim brutalized or worse, family members traumatized, and the legal consequences for the defendant stark and onerous. As with many if not all complex situations that cry out to be understood, the devil is in the details of facts and context, along with a deep understanding of the psychological factors that drove the behaviors in question. When a deific degree claim is presented by the defendant, the forensic situation becomes even more complicated.

In my next blog, I’ll discuss a case that, I think, typifies the complex interplay of circumstance and psychology that jurors and forensic examiners face when insanity is under consideration.


Melton, G. B., Petrila, J., Poythress, N. G., Slobogin, C., Otto, R. K., Mossman, D., & Condie, L. O. (2017). Psychological evaluations for the courts: A handbook for mental health professionals and lawyers. Guilford Publications.

Borum, Randy, and Solomon M. Fulero. (1999). "Empirical research on the insanity defense and attempted reforms: evidence toward informed policy." Law and human behavior 23 (1), 117-135.

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