(A) A person is not criminally responsible for conduct if at the time of the conduct, as a result of mental disease or mental defect, he or she lacks substantial capacity either to appreciate the criminality of his or her conduct or to conform his or her conduct to the requirements of law.
(B) The terms “mental disease” or “mental defect” do not include an abnormality manifested only by repeated criminal or otherwise anti-social conduct.
(ILCS Chapter 720, Act 5, § 6-2) (1981 Code, § 42.06, 6-2)