The mental competence of a defendant charged with crime is assumed in most jurisdictions, and the defense of insanity,whereby the defendant's act is claimed not to have been knowingly and purposefully done because of lack of mental capacity,as indicated is an affirmative defense. (In some jurisdictions,where insanity is suggested, the burden of proving mental competence is placed on the State). The issue, no matter on whom the burden of proof is placed, is what state of mental incapacity must be found by the trier of the facts in order to relieve the defendant from the imposition of the penalties under law for the commission of a public wrong. The question is crystalized in part by the controversy between M'Naghten and Durham.
Lee E. Skeel, M'Naghten v. Durham, 12 Clev.-Marshall L. Rev. 330 (1963)