The defense of insanity is a complete defense to criminal charges only where a defendant meets the substantial burden of proving that he or she was unable to appreciate the nature or wrongfulness of his or her acts due to a severe mental illness or defect. See Tenn. Code Ann. § 39-11-501 (1997). The relevant issue in this case, however, is not whether a mentally retarded individual may be held responsible for a criminal offense, but whether that individual may be executed as a punishment for the offense.