Lindsay Clancy’s murder trial in Massachusetts ended in a mistrial. Had the case been tried in California, however, she would have faced a significantly tougher legal test.

There is little dispute that Clancy was mentally ill when she killed her three young children. Before the killings, she struggled with depression, anxiety, insomnia, paranoia, and suicidal thoughts.

She sought treatment from multiple providers, spent time in a psychiatric hospital, and took numerous medications.

After strangling her children, she attempted suicide by jumping from a second-story window. She survived but was left paralyzed.

That history is heartbreaking. But it was not the central legal question at her trial.

The question was whether Clancy was legally insane when she killed Cora, Dawson, and Callan.

Mental illness and legal insanity may sound like the same thing, but they are not. A person can suffer from a severe psychiatric disorder — even one involving hallucinations or delusions — and still be legally responsible for a crime.

In Massachusetts, a person is not criminally responsible if, because of a mental disease or defect, she could not know right from wrong, or could not obey the law.

That gave Clancy’s lawyers two potential arguments.

First, they could argue that postpartum psychosis prevented her from appreciating that killing her children was wrong, because she delusionally believed she was protecting them, or sending them to heaven.

Second, they could argue that even if some part of her understood that the killings were wrong, a command hallucination or psychotic episode meant she could not control her conduct.

California does not recognize that second argument.

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Under California law, a defendant is legally insane only if a mental disease or defect made her incapable of understanding her actions, or knowing that they were morally or legally wrong. An uncontrollable impulse is not enough.

California also places the burden on the defendant to persuade jurors that it is more likely than not that she was legally insane. Prosecutors do not have to prove sanity beyond a reasonable doubt.

The result is a demanding rule: A defendant may be genuinely ill, hallucinating, or delusional, and still be found legally sane in California if jurors conclude that she understood what she was doing and knew it was wrong.

The evidence strongly suggests that Clancy understood the physical nature of her actions. She used exercise bands to strangle each child separately.

Her California defense therefore would have depended on proving that postpartum psychosis made her incapable of knowing that killing the children was wrong.

Her attorneys would have had powerful evidence. Her psychiatric condition had deteriorated for months. She repeatedly sought help. The defense also maintained that she heard a male voice commanding her to kill them and believed she was sending them to heaven.

Prosecutors would focus on the steps she took before the killings. Clancy asked her husband to leave the house to obtain medication and takeout food. She checked how long the trip would take. While he was gone, she killed the children one at a time in the basement and attempted suicide afterward.

Those details could be viewed as evidence of planning and awareness.

Her attorneys would respond that planning does not prove sanity. A psychotic person can act methodically in pursuit of a delusion.

That is a legitimate and important response — but in California, Clancy would carry the burden of persuading the jury.

Her suicide attempt would also support competing interpretations. The defense could argue that it confirmed the depth of her delusion and despair. Prosecutors could argue that attempting suicide after killing the children showed that she understood what she had done.

None of this diminishes the seriousness of postpartum psychosis. It is a psychiatric emergency that demands greater awareness, faster intervention, and better treatment.

The law is asking a different question.

Mental illness may explain why someone committed a terrible act without legally excusing it.

A California jury could believe that Clancy was severely mentally ill; that the mental-health system failed her; and that she was overwhelmed by suicidal and delusional thinking.

They could still decide that she had not proved legal insanity.

Once convicted by that California standard, Clancy’s mental illness could determine whether she would be committed to a state hospital or sentenced to life in prison. But it would not lead to her acquittal, on its own.

That result might strike some as harsh, but it reflects the narrow purpose of the insanity defense. It is not a general finding that someone was sick, suffering, or behaving irrationally. It is a legal conclusion that mental disease rendered that person incapable of understanding the nature or wrongfulness of the act.

Recognizing mental illness and demanding accountability for the deaths of three children are not opposing positions. A fair legal system must be capable of doing both.

Nathan Hochman is the district attorney of Los Angeles County.