In the trial of Andrea Yates, who drowned her five children in a bathtub last summer, both sides began this week by admitting that she suffered from mental illness.
Yet the prosecution is seeking the death penalty against this Houston mother, while the defense believes she should go free.
How can both sides agree about her mental condition, yet be so diametrically opposed about what to do with her? Such is the legal morass of the insanity plea, a rarely used - and rarely successful - defense.
As the trial opened Monday, Mrs. Yates' lawyers claimed she is not guilty by reason of insanity.
But the standard for proving such a defense is extraordinarily difficult, and even when the legal conditions are met, they aren't often accepted by the jury.
Indeed, when claiming insanity in Texas, the burden of proof is on the defense (unlike other cases where the burden of proof is on the prosecution). And the central question to be answered is: Did the defendant know right from wrong?
"It is a very complex question, what does 'know' mean?" says Richard Bonnie, director of the Institute of Law, Psychiatry, and Public Policy at the University of Virginia in Charlottesville. "It gets at someone's emotional sense, the very nature of their being."
But the prosecution believes it can prove that Yates knew right from wrong. The state's lawyers will point to her talk of thinking about the killings for two years and how she called police immediately after doing so - a sure sign, prosecutors say, that she was aware that what she did was wrong.
The defense will bring up her suicide attempts, her psychotic diagnoses, and her spiraling battle with post-partum depression after the birth of each of her children.
In the end, it will come down to which psychiatrist the jurors choose to believe.
But, as history suggests, advantage is on the prosecution's side.
Surveys show that in the less than 1 percent of the cases where insanity is used as a defense, it is successful only 25 percent of the time.
"Everyday people are naturally skeptical about claims of insanity, so it's always an uphill battle for the defense. And even when someone is clearly mentally ill, juries are not willing to let them walk free. They believe they should be punished," says Mr. Bonnie, who wrote the American Bar Association's new policy on the insanity defense in 1983.
This stricter policy was then passed by Congress in 1984.
The serious and rapid reexamination of the defense began after a jury found John Hinckley not guilty by reason of insanity in the assassination attempt on President Reagan in 1981. He is still in a mentalhospital in Washington, D.C.
So outraged was the public that states began drafting legislation to abolish use of the defense. In the end, it was only outlawed in a handful of states. In most others, the definition was narrowed so substantially that, today, it is almost impossible to prove.
THAT'S partly because "insane" is a legal term, not a clinical one. Studies show that many people suffering from mental illness would be judged "sane" if current legal tests were applied to them.
"Jurors are smart enough to understand [the legal test], but they are uncomfortable with it," says Robert Gordon, a clinical physiologist and director of the Wilmington Institute of Trial and Settlement Sciences in Houston.
"In this Times Square of psychiatry and law, the intersection is a very confusing place. Taking a psychological disorder and applying it to a legal standard doesn't work very well," he says.
While Dr. Gordon says most people believe Yates was mentally ill at the time she killed her five children, he says they also believe it is an explaining condition, not an excusing one. Thus, he thinks the jury will find her guilty of murder.
This rush to punish may not be the only reason for putting Yates behind bars. Many juries simply don't trust the mental-health system.
That point was driven home to E. Fuller Torrey, a psychiatrist who testified for the defense in the case of a homeless man who killed the manager of a Mississippi soup kitchen in the early 1990s. The murderer had been sent to psychiatric hospitals 19 times in the past.
"The jury agreed that this guy was really psychotic and needed to be in a hospital," says Dr. Torrey, with the National Alliance for the Mentally Ill in Arlington, Va. "But they saw how the hospital screwed up 19 times, so they sent him to prison where they knew he wouldn't be a danger anymore."
Torrey believes the jury will consider what will happen to Yates if she is put in a mental hospital instead of prison.
But no matter the verdict, many lawyers and court watchers believe it's a foregone conclusion that Yates will not get the death penalty.
In fact, two of the jurors are psychiatrists whose very job is to find mitigating circumstances in every situation.
"My prediction is that she will be found guilty, but will not get the death penalty," says Gordon.
"Not because she doesn't deserve it," he adds, "but because we [as a society] don't deserve it."