Mich. Comp. Laws § 768.21a

Sec. 21a. (1) It is an affirmative defense to a prosecution for a criminal offense that the defendant was legally insane when he or she committed the acts constituting the offense. An individual is legally insane if, as a result of mental illness as defined in section 400 of the mental health code, 1974 PA 258, MCL 330.1400, or as a result of having an intellectual disability as defined in section 100b of the mental health code, 1974 PA 258, MCL 330.1100b, that person lacks substantial capacity either to appreciate the nature and quality or the wrongfulness of his or her conduct or to conform his or her conduct to the requirements of the law. Mental illness or having an intellectual disability does not otherwise constitute a defense of legal insanity. (2) An individual who was under the influence of voluntarily consumed or injected alcohol or controlled substances at the time of his or her alleged offense is not considered to have been legally insane solely because of being under the influence of the alcohol or controlled substances. (3) The defendant has the burden of proving the defense of insanity by a preponderance of the evidence.