Lack of criminal responsibility — the McHoul standard

Plain-English explainer, drawn only from the case law, the statute, and the model jury instruction. Not legal advice.

The test

A person is not responsible for criminal conduct if at the time of such conduct, as a result of mental disease or defect, they lack substantial capacity either to appreciate the wrongfulness of their conduct or to conform their conduct to the requirements of law. The two prongs are alternatives — either one suffices.

Who has to prove what

The Commonwealth must prove the defendant WAS criminally responsible, beyond a reasonable doubt. The defense does not have to prove insanity. Under Commonwealth v. Lawson (2016), the mere fact that most people are sane is not, by itself, enough to carry that burden once mental-illness evidence is in the case — the Commonwealth may rely on the circumstances of the offense and the defendant's words and conduct before, during, and after.

Does "she knew what she was doing" end the inquiry?: No. The test has two independent prongs. Even a defendant who appreciated wrongfulness is not responsible if disease left her without substantial capacity to conform her conduct to the law. That is the prong defense expert Dr. Zeizel invoked.
If the jury acquits on lack of criminal responsibility, does she walk free?: No. Under M.G.L. c.123 §16, the court may order up to 40 days of hospitalization for evaluation, and the DA or facility may petition for commitment — six months initially, renewable in one-year periods. The model instruction notes a person who remains mentally ill and dangerous "may remain committed for the duration of his [or her] life."
What are the jury's options here?: First-degree murder (life without parole), second-degree murder (life with parole eligibility), not guilty by reason of lack of criminal responsibility, or no unanimous verdict (mistrial; retrial possible). Whether involuntary manslaughter joins the slip is still being argued.

— Commonwealth v. McHoul, 352 Mass. 544 (1967) · Commonwealth v. Kappler, 416 Mass. 574 (1993) (quoting Kostka) · Mass. Model Jury Instruction 9.200 · Commonwealth v. Lawson, 475 Mass. 806 (2016)