1. (a) In any prosecution for an offense, it shall be an affirmative defense that the defendant, at the time the defendant committed the proscribed act or acts, lacked substantial capacity, as a result of mental disease or defect, either to appreciate the wrongfulness of his conduct or to control his conduct within the requirements of the law.

2. (b)

   1. (1) It shall not be a defense under this section if such mental disease or defect was proximately caused by the voluntary ingestion, inhalation or injection of intoxicating liquor or any drug or substance, or any combination thereof, unless such drug was prescribed for the defendant by a prescribing practitioner, as defined in section 20-571, and was used in accordance with the directions of such prescription.
   2. (2) No defendant may claim as a defense under this section that such mental disease or defect was based solely on the discovery of, knowledge about or potential disclosure of the victim's actual or perceived sex, sexual orientation or gender identity or expression, including under circumstances in which the victim made an unwanted, nonforcible, romantic or sexual advance toward the defendant, or if the defendant and victim dated or had a romantic relationship.
3. (c) As used in this section, (1) the terms mental disease or defect do not include (A) an abnormality manifested only by repeated criminal or otherwise antisocial conduct, or (B) pathological or compulsive gambling, and (2) “gender identity or expression” means gender identity or expression, as defined in section 53a-181i.

See Sec. 54-89a re court instructions to jury.

(1969, P.A. 828, S. 13; P.A. 79-49; P.A. 81-301, S. 1; P.A. 83-486, S. 1; P.A. 95-264, S. 64; P.A. 19-27, S. 1; July Sp. Sess. P.A. 20-4, S. 11; P.A. 21-192, S. 12; P.A. 23-19, S. 16.)

History: P.A. 79-49 clarified section by adding provisions concerning effect of use of drugs, intoxicating liquors or combinations of them on defense plea; P.A. 81-301 replaced the provision that “it shall be a defense that the defendant” lacked substantial capacity with “a defendant may be found guilty but not criminally responsible if” he lacked substantial capacity due to mental disease or defect, and replaced “It shall not be a defense under this section” with “A finding of criminal responsibility shall not be barred”; P.A. 83-486 divided section into Subsecs., amended Subsec. (a) by replacing “a defendant my be found guilty but not criminally responsible if” with “it shall be an affirmative defense that the defendant”, and rephrasing parts of said Subsec., amended Subsec. (b) by replacing “A finding of criminal responsibility shall not be barred” with “It shall not be a defense under this section” and amended Subsec. (c) by providing that mental disease or defect does not include “pathological or compulsive gambling”; P.A. 95-264 amended Subsec. (b) to change “licensed” practitioner to “prescribing” practitioner and referenced the definition section (Revisor's note: The reference in Subsec. (b) to “prescribing practitioner, as defined in subdivision (21) of ...” was corrected editorially by the Revisors to “prescribing practitioner, as defined in subdivision (22) of ...”); P.A. 19-27 amended Subsec. (a) by making a technical change, amended Subsec. (b) by designating existing provisions re mental disease or defect proximately caused by voluntary ingestion, inhalation or injection of intoxicating liquor or any drug or substance as Subdiv. (1), adding Subdiv. (2) re discovery or knowledge or potential disclosure of victim's actual or perceived sex, sexual orientation or gender identity or expression, and amended Subsec. (c) by designating existing provisions re terms mental disease or defect as new Subdiv. (1), redesignating existing Subdivs. (1) and (2) as Subparas. (A) and (B), and adding new Subdiv. (2) defining “gender identity or expression”; July Sp. Sess. P.A. 20-4 amended Subsec. (b)(1) by substituting reference to Sec. 20-571(24) for reference to Sec. 20-571(22), effective January 1, 2021; P.A. 21-192 amended Subsec. (b)(1) by substituting reference to Sec. 20-571(28) for reference to Sec. 20-571(24), effective July 13, 2021; P.A. 23-19 made a conforming change in Subsec. (b)(1), effective July 1, 2023.