Criminal Law
Insanity Defense
Definition
The insanity defense excuses criminal liability on the ground that the defendant, at the time of the offense, suffered from a severe mental disease or defect that impaired their capacity for rational conduct. The M'Naghten test (the majority rule) requires that the defendant did not know the nature or quality of the act, or did not know it was wrong. The Model Penal Code test is broader, excusing a defendant who lacked substantial capacity to appreciate the criminality of their conduct or to conform their conduct to the law.
Example
A defendant shoots at a crowd believing they are defending against aliens. If a mental disease prevented them from understanding the nature of their act, the insanity defense may apply.