(a) Affirmative Defense.—It is an affirmative defense in a trial by military commission under this chapter that, at the time of the commission of the acts constituting the offense, the accused, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of the acts. Mental disease or defect does not otherwise constitute a defense.

(b) Burden of Proof.—The accused in a military commission under this chapter has the burden of proving the defense of lack of mental responsibility by clear and convincing evidence.

(c) Findings Following Assertion of Defense.—Whenever lack of mental responsibility of the accused with respect to an offense is properly at issue in a military commission under this chapter, the military judge shall instruct the members as to the defense of lack of mental responsibility under this section and shall charge the members to find the accused—

(1) guilty;

(2) not guilty; or

(3) subject to subsection (d), not guilty by reason of lack of mental responsibility.

(d) Majority Vote Required for Finding.—The accused shall be found not guilty by reason of lack of mental responsibility under subsection (c)(3) only if a majority of the members present at the time the vote is taken determines that the defense of lack of mental responsibility has been established.