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According to Massachusetts case law, a defendant is not criminally liable for his conduct

if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of law.

See Commonwealth v. McHoul, 352 Mass. 544, 546-47 (1967).

How severe must the psychological condition be in order for such a defense to work?

According to the SJC, the essential consideration is not whether a medical professional confirms the existence of a mental disease or defect but rather whether our society should relieve from criminal responsibility a defendant who at the time of the commission of the crime was unable to conform his conduct to the requirements of law because of his condition. See Commonwealth v. Sheehan, 376 Mass. 765, 769 (1978).

Evidence of a defendant’s mental disease (including drug or alcohol addiction) may be used at trial if it is relevant.

[E]xpert opinion evidence pertaining to a defendant’s intoxication or mental impairment is appropriate for a jury to consider when a defendant is charged with a crime requiring specific intent.

Commonwealth v. Okoro, 471 Mass. 51, 66 (2015).

Moreover,

[A] defendant’s inability to form the requisite intent for an element of the crime…could reduce the verdict.

Commonwealth v. Diaz, 431 Mass. 822, 828 n. 4 (2000).

Any indigent defendant who is considering raising a “lack of responsibility” defense should file a pretrial motion with the court, pursuant to M.G.L. c. 261 section 27B, seeking funds for a clinical evaluation.