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
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