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18 U.S. Code § 17 - Insanity defense

(a) Affirmative Defense.—
It is an affirmative defense to a prosecution under any Federal statute 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 and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense.
(b) Burden of Proof.—
The defendant has the burden of proving the defense of insanity by clear and convincing evidence.
(Added Pub. L. 98–473, title II, § 402(a), Oct. 12, 1984, 98 Stat. 2057, § 20; renumbered § 17, Pub. L. 99–646, § 34(a), Nov. 10, 1986, 100 Stat. 3599.)