Tennessee Code Title 39. Criminal Offenses § 39-11-501

(a) It is an affirmative defense to prosecution that, at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature or wrongfulness of the defendant's acts. Mental disease or defect does not otherwise constitute a defense. The defendant has the burden of proving the defense of insanity by clear and convincing evidence.

(b) As used in this section, “mental disease or defect” does not include any abnormality manifested only by repeated criminal or otherwise antisocial conduct.

(c) No expert witness may testify as to whether the defendant was or was not insane as set forth in subsection (a). Such ultimate issue is a matter for the trier of fact alone.