I’ve spent years writing about cases of mothers who kill their children, so when I heard about Lindsay Clancy’s arrest in connection with the deaths of her three children, my hunch was that she was insane. As a legal scholar, I also knew she’d face an uphill battle to prove it.

On the day that Ms. Clancy killed her children in 2023, she built a snowman with her family and ordered takeout. While her husband was picking up dinner, she strangled her children, who were 5 years, 3 years and 8 months old, before jumping out of her home’s second-story window.

She told a forensic psychologist that a voice ordered her to do it. Her lawyer has argued that she was suffering from postpartum psychosis, a condition driven in part by hormonal shifts after childbirth that can cause hallucinations, delusions and paranoia, along with heightened risks of suicide and infanticide. I am not a doctor and haven’t met Ms. Clancy, but from everything I’ve read about the case, I believe this diagnosis is probably accurate.

Because the symptoms of postpartum psychosis wax and wane, mothers can seem to be coping when, in fact, they are drowning. This on-and-off psychosis also makes it difficult for women who kill their children to qualify for the insanity defense, which is often the only defense available to defendants in cases like Ms. Clancy’s.

The question her jurors must answer is whether her illness rendered her unable to appreciate that it was wrong to kill or incapable of stopping herself from doing it. That framing means that in cases like this, prosecutors point to any signs of competence or control or even the fact that women are horrified by their actions to argue that they should not qualify for the insanity defense.Cases like Ms. Clancy’s are more common than we might want to believe. A 2020 KQED investigation found more than 100 women in California prisons for killing their children, a majority serving life sentences. Roughly 40 percent of those cases involved a child under the age of 1 — meaning the mother might have been in a period when women are at higher risk for postpartum mental illness.

I have reviewed many such cases and have met some of those women. Some will never recover their sanity — a blessing of sorts. Those who do recover feel overwhelming guilt and pain. During the medical examiner’s description of the children’s fatal injuries, Ms. Clancy began sobbing uncontrollably, moaning, “I can’t do this.” She was so distraught that the judge ordered a recess.

It seems clear to me, from the evidence presented so far, that Ms. Clancy should be acquitted by reason of insanity. She deserves mercy because she did something she never would have done if she hadn’t been terribly ill. Even though we know that symptoms of postpartum psychosis can vary wildly from hour to hour, our existing legal framework asks jurors to determine the defendant’s exact state of mind at the moment of killing. This is an impossible task, especially in cases like Ms. Clancy’s; she has said she doesn’t fully remember her acts. Then we give jurors a stark choice, either full acquittal (for reasons of insanity) or conviction of first-degree murder (almost always with life imprisonment). Facing the horror of these killings, many juries opt to convict.

The standard formulation of the U.S. insanity defense dates back to the mid-19th century, when we understood so little about mental illness that expert witnesses sometimes still opined on defendants’ criminal propensity by measuring their skulls. The so-called M’naghten rule originated in England after the 1843 acquittal of Daniel M’naghten, who killed the prime minister’s secretary while suffering from an insane delusion.

Responding to outcry against the not guilty verdict, the House of Lords, along with the common law judges of the time, established a test that largely limited the insanity defense to cases in which the defendant lacked the capacity to tell right from wrong. Under this stricter rubric, someone like M’naghten — who fired a gun with the intention to kill and whose behavior (concealing his guns, loitering near the area for weeks before the shooting and lying about why) indicated he might have known it was wrong — would not qualify as insane. Never mind that he was flagrantly out of touch with reality.

In a sense, Ms. Clancy is lucky, because Massachusetts is one of a handful of states that have updated their insanity defense. Four states don’t allow it at all. Most U.S. states still follow the M’naghten rule, restricting insanity to cases in which defendants did not know their actions were wrong. By contrast, for more than 50 years, Massachusetts has held that defendants should not be judged criminally responsible if mental illness renders them unable to control their actions to follow the law.

Yet the entwining of lucidity and delusion in postpartum psychosis complicates the jury’s task. To the outside observer, Ms. Clancy appeared functional, chatting at preschool pickups and taking care of her children. But the journal in which she wrote about her symptoms and medications tells of her frantic efforts to regain control over her mind. In the four months before the tragedy, she was prescribed 13 medications by different providers. She wrote about having “horrible thoughts” and made Google searches related to schizophrenia, hallucinations, psychosis, bipolar disorder and sociopathy.

In its opening statement, the prosecution pointed to Ms. Clancy’s apparent competence, using it to suggest that there was no evidence of psychosis. Assistant District Attorney Shanan Buckingham argued that Ms. Clancy was acting “pretty normal” on the day of the killings and pointed to the fact that she had mapped out her husband’s route, allowing her to determine how long she would have to commit the crime.

When the jurors in the trial start their deliberations, which could happen as soon as this week, they may well struggle with which version of events to believe. But our choices in cases like Ms. Clancy’s needn’t be so constrained.

Britain’s Infanticide Act of 1938 recognizes that cases involving mothers who kill infants under the age of 1 are often different from other intentional killings. Instead of trying these women for murder and judging them under an insanity defense, it allows them to be charged with infanticide and to be punished as if convicted of manslaughter. Prison sentences are rare; usually the women are sentenced to probation and counseling. Other countries have similar laws, including Canada and parts of Australia.

It’s not a perfect solution; surely some of the mothers should have been found not guilty for reasons of insanity. And perhaps some are even truly guilty of murder. Still, the result of the British law is, on the whole, far more just and merciful than the life sentences so common in the United States. And because these cases typically don’t go to public trial, tragedies can avoid becoming true-crime dramas. We should follow Britain’s example.

Once upon a time, a U.S. judge might have tempered the cruelty of a guilty verdict by exercising discretion at sentencing. Today many states, including Massachusetts, prohibit judicial discretion in sentencing in cases like these. If Ms. Clancy is convicted, her judge must sentence her to life without parole.

Ms. Clancy’s trial, with its high-stakes drama and surrounding media frenzy, does little to honor her children. A just society punishes only the blameworthy. Our approach to insanity in these cases makes it harder, not easier, to do justice.

Michelle Oberman is a professor at Santa Clara University School of Law and an author of “When Mothers Kill: Interviews From Prison” and “Mothers who Kill Their Children.”

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