Criminal insanity refers to a mental illness or disease that makes it impossible for a defendant to know they were committing a crime or to understand that their actions are wrong. A defendant found to be criminally insane can assert an insanity defense. Insanity functions as a failure of proof defense where the defendant admits to having committed the wrongful act, but claims but argues they are not culpable because of their mental defect prevented them from establishing the required mental state.
Tests to determine if a defendant is criminally insane vary from state to state. For instance, any jurisdiction that follows the Model Penal Code (MPC) rule looks to see if the defendant lacked both substantial capacity to appreciate the wrongness of their actions and substantial capacity to conform their actions to the law. Jurisdictions that follow common law tests are primarily split between the M'Naghten Rule and the irresistible impulse test. Under the former, a party is criminally insane if they lacked the capacity to know they were committing a crime due to a mental defect. Under the latter, a defendant is criminally insane if they lacked total capacity to conform with the law.
The first use of the insanity defense in the United States took place in Cayuga County, in New York State in People v. William Freeman (1847).
Notably, states have wide freedom in how they wish to allow the insanity defense. As seen in Kahler v. Kansas, due process does not require a state to adopt a criminal insanity test that considers whether the defendant recognized that their crime was morally wrong. In other words, states are not required to acquit a defendant who believed their actions were morally just.
[Last updated in July of 2022 by the Wex Definitions Team]