The words “insane” and “insanity” are legal, rather than medical, terms. Contemporary legal definitions of insanity are derived from the M’Naghten test, formulated by the House of Lords in 1843, which set out the test as whether (a) the defendant knew what he was doing, and (b) if so, knew that it was wrong. It was adopted by the U.S in 1851, and remains the test in about half of the states. A handful of states–currently Idaho, Kansas, Montana, Utah and Vermont–do not allow for an insanity defense.