In pleading not guilty by reason of insanity, James Holmes is trying to do something no accused mass shooter in America has done in more than 20 years: win a mental-health case.

He will do it in a state that is one of only a handful in the country to put the burden of proving a defendant’s sanity on the prosecution. Even so, Holmes faces long odds for a defense that studies show is raised in only about 1 percent of all felony cases nationally and successful in only about a quarter of those.

Nearly 11 months after the attack on the Century Aurora 16 movie theater in which Holmes is accused of killing 12 people and wounding 58 others by gunfire, the case against Holmes has now reached its most critical moment. The next two months — during which Holmes will undergo an evaluation at the state Mental Health Institute and be under near-constant surveillance for clues to his sanity — could very likely determine the outcome of the case.

“The chances of being successful are really, really slim,” said Michael Perlin, a professor at New York Law School.

Mass shooters very rarely survive their crimes to face court charges. According to a database of American mass shootings in the last 30 years compiled by the magazine Mother Jones, suspects in only 19 of 61 mass shootings examined by the magazine were arrested. Others either committed suicide or were killed.

Of those 19, only four — including Holmes — pleaded insanity.

The mass shooting in which the alleged killer won an insanity case was in 1988, said Grant Duwe, a Minnesota correctional official who is the author of the book “Mass Murder in the United States: A History.” That man, Michael Hayes, said God told him to go on a shooting spree that left four people dead and five wounded in North Carolina.

Among the 1,186 mass killers going back decades that Duwe has analyzed in his research, only three mass shooters — and 21 mass killers, in total — have successfully raised insanity defenses. That’s less than 2 percent.

“The evidence is consistent with what we see about the success rate for defendants who use the insanity plea in general,” Duwe said. “Very few defendants are successful in using the insanity plea in general. And that extends to mass murder.”

The reason for the low success rate is the high bar that laws set for insanity. In Colorado, it is not enough for defendants to be merely mentally ill. Instead, the law defines insanity as having judgment so impaired by mental illness that the defendant could not tell right from wrong.

The linchpin in determining insanity is the intensive mental-health evaluations that a defendant pleading insanity is required to undergo. The judge overseeing the theater shooting murder case this week ordered Holmes to be evaluated at the Colorado Mental Health Institute in Pueblo.

Dr. Neil Gowensmith, who teaches in the University of Denver’s Graduate School of Professional Psychology, said mental-health evaluators will spend hours with a defendant and even more hours poring over case reports, mental-health history forms, witness statements and other evidence before making a diagnosis. Information as close as possible to the time of the crime — a journal entry right before, for instance, or a statement immediately after — is especially valuable.

Evaluators will look for details showing that defendants knew what they were doing was wrong or tried to cover up evidence of their crime. They will look for logical or illogical chains of behavior, whether defendants acted in a way where A leads to B leads to C.

“For the psychotic person, A might kind of lead to D without making much sense,” Gowensmith said. ” … The hallmark of a psychotic disorder is the lack of insight.”

When a Denver jury examined the claim of Edward Romero that he was insane when he killed 16-year-old Alicia Martinez, attention focused on how Romero shot Martinez through a couch cushion, concealing the sound. The jury rejected Romero’s insanity defense and convicted him of first-degree murder.

But in the case of Deer Creek Middle School shooter Bruco Eastwood, jurors heard about Eastwood’s delusions that mutants were invading his body and imaginary creatures were stealing from his stomach. A psychiatrist hired by the prosecution said he saw evidence that Eastwood knew what he was doing was wrong — for instance, in a statement Eastwood made that he knew he would go to prison for a long time. The jury found Eastwood not guilty by reason of insanity.

In Holmes’ case, it appears evaluators will have a wealth of information. There is the notebook Holmes mailed to his psychiatrist just prior to the shooting — and the burned money and sticky note with an infinity symbol it contained. There was Holmes’ posting on a dating website prior to the shooting, “Will you visit me in prison?” There was Holmes’ prompt request for an attorney when police questioned him. And there was Holmes’ bizarre puppet gestures when detectives placed bags over his hands to preserve evidence.

It is not uncommon for evaluators to disagree on a diagnosis, Gowensmith said. But, in 196 cases that Gowensmith looked at during his previous job in Hawaii, he found that doctors agreed three-quarters of the time on whether a psychotic disorder was present. In cases where there is consensus, juries agree with the evaluations as much as 90 percent of the time, according to studies written by Perlin, the law school professor.

“It’s not like a blood test,” Gowensmith said, “but the reliability is reasonably high.”

Colorado’s law, like many others across the country, is based on what is known as the “M’Naghten rule” — named after an Englishman who shot and killed the prime minister’s secretary in 1843. What makes Colorado’s law somewhat unique is that prosecutors must prove a defendant is sane.

Thirty-five states put the burden of proving insanity on a defendant. Four states have abolished the insanity defense altogether or have a “guilty but insane” verdict.

The prosecution’s burden has been in Colorado law for generations and was upheld by a 1968 state Supreme Court ruling that remains precedent.

Several lawmakers tried this legislative session to again shift the burden to the defendant. A legislative committee rejected the bill.

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold