The question at the heart of a closely watched Massachusetts murder trial is not whether a mother killed her three young children, but whether mental illness means she can’t be held legally responsible.

Lindsay Clancy, 36, is charged with three counts of murder in the Jan. 24, 2023, killing of Cora, 5; Dawson, 3; and Callan Clancy, 8 months. The prosecution says Clancy planned and executed the murders by sending her husband, Patrick Clancy, out of the house on errands, then strangling the children with exercise bands in the basement of their Duxbury home.

Clancy admits she strangled her three children, then cut her own throat and neck and jumped from a second-story window, leaving her paralyzed. Her defense attorney, Kevin Reddington, argues that Clancy is not guilty of murder because she was experiencing a psychotic break induced by postpartum psychiatric disease aggravated by overmedication.

Legal analyst and author Margaret McLean, a former Essex County prosecutor and Boston College law professor, told the Patriot Ledger, part of the USA TODAY Network, the case boils down to one question.

“Was this woman, Lindsay Clancy, criminally responsible?” McLean said.

Why the prosecution faces a special challenge in Lindsay Clancy case

A defendant is not criminally responsible for their conduct if they have “a mental disease or defect, and as a result of that mental disease or defect, lacks the substantial capacity either to appreciate the criminality or the wrongfulness of her conduct or to conform her conduct to the requirements of the law,” according to a definition read in court by Judge William Sullivan.

In most states, the defense has to prove a lack of criminal responsibility, more commonly known as an insanity defense, but Massachusetts shifts that burden onto the prosecution, McLean explained. That means the prosecution must convince the jury beyond a reasonable doubt that Clancy was capable of making responsible decisions at the exact time of the killings, she said.

“That’s a big burden to prove in this case beyond a reasonable doubt,” McLean said. “I think it will be a Mount Everest. I do.”

Prosecutors called more than 70 witnesses to the stand including Clancy’s husband, family friends, first responders and medical professionals, in an effort to surmount this burden before resting their case on Aug. 17. Plymouth County Assistant District Attorney Shanan Buckingham called Clancy a “controlling, meticulous and manipulative” mother who committed the murders because she could no longer control “every little detail” of her and her family’s life.

When it came to Clancy’s mental health treatment in the months prior to the killings, Buckingham also alleged that she “chose to manipulate her providers” and “seek out the quick and easy fix when she was feeling depressed and anxious.”

“This is not a public debate about women’s mental health and how the medical system treats women. This case is about this defendant, Lindsay Clancy,” she said.

Psychiatrists and psychiatric nurse practitioners testified about treating Clancy at different points in the months before the killing and the various medications she was prescribed. Prosecutors questioned Patrick Clancy about her efforts to get mental health treatment and played the heart-wrenching 911 call he made in January 2023 when he found his children for the courtroom.

Asked which side she believes has the greater challenge, McLean said the prosecution.

“Even though there’s all this horrible evidence, they still have to prove beyond a reasonable doubt that Lindsay Clancy was legally responsible and legally sane at the exact moment of the offense,” she said.

What the defense will argue about postpartum psychosis

Though he agreed the prosecution bears the greater burden, Chris Slobogin, director of the criminal justice program and affiliate professor of psychiatry at Vanderbilt University, told USA TODAY succeeding with an insanity defense won’t be easy for Clancy’s attorney. Slobogin said juries tend to be highly skeptical of such defenses, and past research has found the insanity defense only worked in about 25% of cases.

“Whether in fact the switch of the burden to the prosecution makes a big difference is not at all clear,” he said. “In fact, most research shows, it doesn’t matter too much who the burden is on.”

In his opening statement, Reddington detailed Clancy’s struggle with postpartum depression, anxiety and psychosis for the jury. He alleged that she heard a male voice telling her to commit the killings, and described her as a doting mother who “tried very hard to get the help she needed.”

“These children were loved and nurtured and cared for by her,” he said. “She didn’t want to get rid of her kids because of her selfish life as the government indicates. She loved those children right up to the time of their death.”

The defense has called witnesses including Clancy’s mother and sister, who testified about her declining mental health prior to the killings. Reddington also questioned Clancy’s former colleague, an emergency medicine doctor and a psychologist who testified that he believed Clancy to be suffering from psychosis at the time of the killings.

“She had no appreciation for the wrongfulness of her act,” psychologist Paul Zeizel said.

Through expert testimony, Reddington will have to convince the jury that mental disease and misprescribed medications caused a psychotic break on Jan. 24, 2023, and Clancy therefore cannot be held criminally responsible, McLean said.

“Reddington will argue that she snapped right before she killed them,” she said. “He’s going to have a hard time explaining exactly when this occurred. That’s tough.”

What happens if Lindsay Clancy is found not criminally responsible?

Clancy is facing three counts of first-degree murder, which in Massachusetts carries a mandatory sentence of life in prison without parole.

Under Massachusetts law, if a defendant is found not guilty or lacking criminal responsibility, they are typically committed to a state hospital for mental health treatment and evaluation.

Clancy has been held at Tewksbury Hospital, a facility in Tewksbury, Massachusetts, for court-ordered mental health treatment since May 2023.

Ultimately, the jury will have to reach a unanimous verdict on the question of criminal responsibility. McLean said the nature of the case will make it difficult for 12 people to reach a consensus.

“It’s going to be a battle in that in jury room,” she said.

(This story was updated to add new information.)