Generally speaking, criminal insanity is understood as a mental defect or disease that makes it impossible for a defendant to understand their actions, or to understand that their actions are wrong. A defendant found to be criminally insane can assert an insanity defense. When asserting an insanity defense, the defendant essentially admits to having committed the wrongful act, but claims that they are not culpable because of their mental defect.
It is important to recognize that the definition of criminal insanity can vary depending on the jurisdiction, meaning that one state may define insanity differently than another. For instance, some jurisdictions use the irresistible impulse test, where defendants essentially assert that their mental defect made it impossible to resist their impulses. Additionally, jurisdictions may follow the Model Penal Code, or the more common “M'Naghten rule.”
Recently, the Supreme Court upheld Kansas’s understanding of the criminal insanity defense, finding that 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, due process not require that a court acquit a defendant who understood their actions, even if that defendant believed their actions were moral. To read the decision in full, see Kahler v. Kansas.
[Last updated in May of 2020 by the Wex Definitions Team]