(a) A defendant is not criminally responsible for criminal conduct if, at the time of that conduct, the defendant, because of a mental disorder or mental retardation, lacks substantial capacity to:

(1) appreciate the criminality of that conduct; or

(2) conform that conduct to the requirements of law.

(b) For purposes of this section, “mental disorder” does not include an abnormality that is manifested only by repeated criminal or otherwise antisocial conduct.