A Massachusetts judge on October 1 refused to acquit Lindsay Clancy outright, weeks after her murder trial ended in a mistrial on September 4. Clancy's lawyers argued she lacked criminal responsibility because of postpartum psychosis. The jury could not reach a unanimous verdict, and she is due back in court on November 2.
The case has renewed an old question. How does the insanity defense work, and how often does it succeed? The short answer is that it is raised rarely and wins less often than many people assume.
The tests states use
There is no single American test for legal insanity. The best known is the M'Naghten rule. Under that rule, as the Legal Information Institute at Cornell Law School explains, defendants are presumed sane. To be excused, they must show that a mental condition left them unable to know what they were doing, or that they knew what they were doing but did not know it was wrong.
A broader test comes from the Model Penal Code. It excuses a person who, because of mental disease or defect, "lacks substantial capacity either to appreciate the criminality" of the conduct "or to conform his conduct to the requirements of law," according to Cornell's summary of the standard. The institute says 21 states have adopted that language in whole or in modified form. The test does not cover a condition shown only by repeated criminal or antisocial conduct.
Massachusetts uses language close to that test. In the Clancy case, prosecutors had to prove she did not lack "substantial capacity to appreciate the criminality or wrongfulness of her conduct," Fox News reported. Judge William Sullivan wrote that "Mental illness alone does not preclude a finding of criminal responsibility beyond a reasonable doubt."
Federal courts use a narrower test. Under the Insanity Defense Reform Act of 1984, a federal defendant must show that, as a result of "a severe mental disease or defect," he could not appreciate the nature and quality or the wrongfulness of his acts. The defendant has to prove it by "clear and convincing evidence," according to the institute's background page on the federal standard.
Some states go further. In 2020 the Supreme Court ruled 6 to 3 in Kahler v. Kansas that a state does not violate due process by dropping the traditional moral wrongfulness test. Kansas abolished the traditional defense in 1995. Defendants there can argue only that mental illness kept them from forming the intent the crime requires, and mental health evidence can be raised at sentencing, SCOTUSblog reported.
Justice Elena Kagan wrote that "due process imposes no single canonical formulation of legal insanity." Justice Stephen Breyer, in dissent, said Kansas had "eliminated the core of a defense that has existed for centuries." Kansas is one of five states that have in practice eliminated the insanity defense, public radio reported at the time.
Rare, and rarely successful
The defense is far less common than its reputation. A widely cited eight-state study published in 1991 found it was raised in about 1 percent of felony cases. Only 26 percent of the defendants who raised it were found not guilty by reason of insanity.
More recent estimates land in the same range. Michael Benza of Case Western Reserve University School of Law told Fox 8 in 2025 that the plea is filed in about 1 percent of criminal cases and succeeds in roughly 25 percent of them.
A successful defense also does not mean walking free. It usually leads to confinement in a state hospital for psychiatric treatment, the institute notes. In Clancy's case, a verdict of not guilty by reason of insanity would send her to a state psychiatric hospital for evaluation, with commitment subject to ongoing court review, Fox 9 reported.
What comes next for Clancy
Prosecutor Shanan Buckingham told the court that "People can still have criminal responsibility despite a mental illness," Fox 9 reported. Clancy's lawyers, led by Kevin Reddington, argue that retrying her would violate protections against double jeopardy. The Plymouth County District Attorney's office has not said whether it will try the case again.
For background, see our coverage of the postpartum psychosis defense and the 11 to 1 hung jury. Daniel Rosenberg, Michael Dye and Lloyd Golburgh walked through the evidence on both sides in Episode 85, and Rosenberg and Dye broke down the deadlock in Episode 90.
This article is general information about how the law works, not legal advice.
Two Taps News stories summarize reporting and the analysis discussed on Two Taps and Friends. They are not legal advice. Read our standards.
