'Brain' Defense Tests Notions of Guilt ; Lawyers in the Yosemite Murder Case May Say Their Client Has a Condition That Caused Him to Kill

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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. …