This Article investigates jurisdictions’ compliance with M’Naghten’s directive for how to treat delusions in insanity cases and assesses the validity and reasonableness of courts’ application of the law. Most U.S. jurisdictions employ an insanity test roughly modeled on the rule articulated in the 1843 M’Naghten’s Case. This test focuses on a defendant’s inability to know, because of a mental disease, the nature of her act or its wrongfulness. But the M’Naghten judges also issued a second rule — particular to delusions — that has received much less attention. This rule holds that, when the defendant labors under a “partial delusion only,” her culpability must be assessed as if the factual content of her delusion were true. Thus, if a person with delusions killed as she believed in self-defense, she should be acquitted. But if she killed anticipating future harm, she would be convicted of intentional murder. Commentators have long dismissed the delusion rule as obsolete, and the last examination of states’ use of the rule was sixty years ago.

This Article excavates the insane delusion rule and assesses its current force. Its review reveals the rule maintains its vibrancy, continues to evolve, and in some places is growing in influence. Nine jurisdictions — California, Florida, Georgia, Nevada, Oklahoma, Tennessee, Texas, and the federal and military systems — give special significance to delusions. These jurisdictions vary in their understanding of how the rule relates to general insanity; whether the rule functions only to establish (not defeat) insanity; and whether it operates as a background principle or manifests in jury instructions. The status of the rule is currently in flux, so understanding its permutations may inspire movement in the law.