Cary Stayner has already confessed to four murders in and around Yosemite National Park in February 1999 - a particularly gruesome killing spree that horrified America.
Yet when he goes to trial this October, where he could face the death penalty, his defense lawyers may try to plant doubt in jurors' minds - not about their client's guilt, but about whether his brain has an abnormality that made him predisposed to murder.
It would be a controversial proposition, but other defense teams have used it increasingly in recent years - sometimes successfully - to try to get life sentences, instead of execution, for clients.
To some, such a defense is part of a trend toward "medicalizing" evil, an effort to find psychiatric or biological reasons for violent criminal behavior. In it, critics see a way to reduce individual responsibility for criminal behavior - of asserting that free will or evil motive is in some measure subordinate to a medical condition.
Supporters, though, say such conditions cannot be completely discounted, especially in capital cases. Medical research indicates that health problems can cause some people to behave irrationally - even insanely, and these conditions should be taken into account in determining a fit punishment, they say.
To make their case, defense lawyers are relying on a form of brain-imaging technology most frequently used to study and treat illnesses that are considered brain-related, such as epilepsy.
The procedure, known as positron emission tomography, or PET scans, produces brightly colored images that highlight activity, or lack of it, in brain cells. Some defense lawyers have used those results to convince juries that a murderer was not capable of controlling murderous behavior. (Stayner underwent these tests last month, but his lawyers won't say if they will be used in court.)
There's just one problem: Many neuroscientists and criminologists argue that PET scans cannot prove any correlation between brain abnormalities and criminal behavior.
"It's right up there with the 'Twinkie defense,' " says Franklin Zimring, a law professor at the University of California, Berkeley, referring to a defense strategy used in 1979 to persuade a jury that a defendant's mental capacity was diminished - by the large amount of junk food in his diet - when he shot and killed San Francisco Mayor George Moscone and county Supervisor Harvey Milk.
"Obviously, biology has a tremendous impact on behavior. But just as obviously, there are about a million steps between anything in biology, other than gender, and any sense of behavioral proclivities that are as specific as violent crime," he says.
Neuroscientists say brain research offers nothing conclusive about why certain people commit violent crimes - that such behavior depends on a huge range of factors, from biological factors to drug and alcohol abuse.
"We're finding out a lot of things about the brain," says Evan Balaban, a neurobiologist at the Neurosciences Institute in southern California. "But don't mistake that for a real understanding of what's going on. In the area of brain functions and what causes people to commit murder, I think biology has little to say. And in the future I think it will still have very little to say."
PET scans, in use since the 1970s, have been considered a crucial technology in allowing scientists to better explore the brain, and draw observations about it, by charting brain-cell activity. It wasn't until the mid-1980s, however, that PET scans came into use in murder trials.
Unlike other techniques, such as hypnosis, no professional guidelines govern the use of PET scans in court. But observers say PET scans have been involved in scores of cases over the past several years - often where a death penalty was at stake.
"It's in vogue," says Mike Rustigan, a criminologist at San Francisco State University and a critic of the use of PET scans. "It's highly visual, which is tailor-made for the courtroom. You come in with your slides and make a very 'scientific' presentation. [Juries] are easily persuaded, because it looks like science."
Christopher Plourd, a criminal defense attorney in San Diego, used PET scans in a case in the early 1990s to persuade jurors that a client deserved life imprisonment instead of execution. The man had burglarized and beaten an elderly man to death. His guilt was never in question.
But when it came time for sentencing, Mr. Plourd introduced evidence that his client had suffered a brain injury as the result of an accident nearly 20 years earlier. He brought in witnesses who testified that before the accident, the defendant had behaved normally, and that afterward, he had become increasingly violent. Plourd showed the jury a PET scan that depicted the damaged section of his brain. The jury voted for life imprisonment.
"A PET scan is very helpful," says Plourd. "The jury ... can accept it more readily than just words or the testimony of an expert."
EVEN advocates of allowing PET scans in court, however, warn that the tests can be used in misleading ways. Michael Welner, a forensic psychiatrist who has testified for both defense and prosecution attorneys, says the scans can offer legitimate findings only when they are supported by a clinical history that documents disturbed behavior.
"It's a dangerously exploitable technology," says Dr. Welner, chairman of the Forensic Panel, a group of psychiatrists and psychologists who provide peer-reviewed testimony in criminal cases. "The jury doesn't know better. It's an area ... ripe for distortion."
But like other PET scan supporters, Welner argues that the tests - used properly - are important during the sentencing phase of death-penalty trials. By law, defense lawyers must present any evidence of "mitigating factors" that may lead jurors to opt for a lighter sentence.
Critics like Mr. Rustigan, however, argue that PET scans are simply the latest tool being used to excuse violent criminal behavior, in some measure, based on psychiatric or biological reasons.
"There's this very great effort, especially in psychiatry, to medicalize evil, to come up with some new syndrome to explain bad behavior ... that through the centuries, back to the Old Testament and to Greek epics, has always been called evil," he says. "The social impact is to lessen responsibility to some extent.
"Now, instead of saying, 'the devil made me do it,' " he says, "we're saying, 'my chemistry made me do it, my brain abnormality made me do it.' "
(c) Copyright 2000. The Christian Science Publishing Society