In an insanity defense, the defendant admits the action but asserts a lack of culpability based on mental illness. Contact us today to learn more.Many states define legal insanity according to the M'Naghten test, developed in an 1843 English case. Is Legal Insanity Different Than Psychological Insanity? What Happens If I Plead Insanity? The insanity defense is an affirmative defense to criminal charges in the state of New York. A defendant who intends to assert a defense of insanity at the time of the alleged offense must so notify an attorney for the government in writing. The Criminal Defense of Insanity. The defendant must convince the judge or jury that his insanity was highly probable.