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.

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 in cases like Ms. Clancy’s. 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.

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. The standard formulation of the U.S. insanity defense dates back to the mid-19th century, when we understood so little about mental illness. Under this stricter rubric, someone like M’naghten—who killed the prime minister’s secretary—fits, but the law fails to capture the reality of postpartum psychosis.