Special 21stC 
home page

DNA as a forensic instrument


IN A NORTH CAROLINA courtroom in June, a man was acquitted of murder after spending 11 years in prison. Another inmate had boasted of committing the crime, and DNA tests showed the braggart had, indeed, been at the scene.

DNA test results, first allowed as evidence in 1990, are changing the criminal justice system, but not always for the better. Reliance on DNA typing risks perpetrating injustice by focusing an entire case on physical evidence, downplaying fundamental aspects of a trial: motive, the quality of police work, the presumption of innocence, and considerations of reasonable doubt. When considering the genetic determination of criminal conduct, warns Harold Edgar, professor of law and technology at Columbia Law School, "We must be careful not to reduce complex human behavior to biological simplicities."

Genetic evidence is introduced in fewer than 1 percent of prosecutions, yet these cases involve the most heinous acts-usually murder or rape-and the tests often have a dramatic effect on verdicts. When DNA profiles were first presented in court, skeptics wondered whether they could uniquely identify a person. Now, Edgar says, judges and juries readily accept the data, though mistakes in the collection and testing of samples do occur.
"Juries think science is so absolute that DNA evidence is without question," says Charles Leonard, a partner at Tremper, Bechert & Leonard in Fort Wayne, Ind., and a court-appointed defender in Indiana v. Hopkins, the third case involving DNA evidence ever tried in the country. A woman had been raped and murdered; police fingered Leonard's client. The evidence produced was weak, however, and the jury was unconvinced, according to Leonard. Then the prosecutor ordered DNA tests, the judge admitted the results, "and the guy was convicted, like that."

The power of DNA evidence lies in statistics. Certain chromosome sites contain unique sequences of base pairs. Forensics technicians typically analyze three to five of these sites, called polymorphic markers, yielding a probability that only one person in, say, 100,000 or 1,000,000 could have the same profile. As a defense attorney, Leonard says, "You fight like hell to keep the statistics out, because once they're introduced they almost replace a jury's thinking."

Leonard says fundamentals such as motive cannot take a back seat to physical evidence. "Let's say a husband finds out his wife is involved with the gardener," he suggests. "He becomes enraged. He gets the gardener's hair and blood from some yard tools. He leaves town, sneaks back in, kills his wife, and plants the samples at the scene. Investigators find the gardener's DNA. So what? The question is how it got there."

Jurors, Edgar says, must remember: "Just because physical evidence is found, that doesn't mean a defendant is guilty. That's what O.J. Simpson's lawyers were trying to do: to get the jury to see that DNA is just evidence that can be mishandled or even planted. There can be plenty of room for doubt."

THE ISSUES SURROUNDING genetic information in trials may soon become more complicated. The next likely controversy will concern the science of population genetics. Even if a combination of markers is rare among all people, it might appear at higher rates in some ethnic subgroups, says Conrad Gilliam, professor of genetics and development at Columbia. He testified in the 1990 case Castro v. New York State, the first in which a prosecution's attempted use of DNA data was thrown out by the court.

Suppose a murder is committed in Chinatown, Gilliam conjectures, and the police find blood samples. Certain polymorphic variants that occur frequently in Chinese people are rare in Caucasians. If these markers show up in the sample, and the police produce a Chinese suspect, a prosecutor could try to use the DNA as further evidence against him. "However," Gilliam says, "a defense attorney could argue that there could be so many local suspects with the same profile that the evidence has no bearing on the case."

Other future tactics may be even more controversial, calling upon work that borders on pseudoscience: arguments that genes predispose a defendant to violent behavior. These scenarios are not far from reality. In Indiana, a middle-aged man whose father had died of Huntington's disease was convicted of murdering his pregnant sister-in-law. The man had never been tested for the HD gene, but HD usually appears at mid-life and can cause violent outbursts. The man's attorney wanted him to be tested and introduce a genetic defense, but the grief-stricken defendant refused to put up the fight. (Depression is another symptom of the disease.)

The point of impact for the predisposition argument may be in sentencing, Edgar says. An attorney could argue for leniency because his client's genes caused him to have diminished responsibility for the crime. Edgar notes that the argument could backfire, since society might feel safer with such a person kept off the street.

Trying to explain behavior through physical characteristics is simplistic, yet society does it all the time, says Robert Wachbroit, a philosopher at the University of Maryland. "People say, 'Look at his face; he must be a criminal.' Drawing such conclusions from the genetic data is even more problematic. When people do that, they ignore the role of environmental factors."

Why, then, does DNA evidence carry so much weight with a jury? "Certain things capture the public consciousness," Wachbroit explains. "In the 1920s and 30s, when research in blood typing was advancing, there were many attempts to predict behavior based on blood type. Researchers so much wanted to find links that work in this area continued for years, until it was widely recognized to be worthless. A few decades later, the enthusiasm lay in trying to explain everything in terms of upbringing and environment. Now we're back to thinking that the main considerations must be biological. It's as if we're on a pendulum."

Lay jurors may lack the understanding needed to see through a smoke screen. "I give my lectures on DNA to Ph.D.s in other fields, and it's hard for them to absorb it all," Gilliam notes. "The average juror would need a two-week course to be able to judge any comment on DNA evidence." The scientific community has tried to help by convening scientists, attorneys, judges, and politicians to set standards for the types of genetic information or arguments that would be admissible in court. But each time the effort has fallen apart.

Science cannot yet provide conclusive results on genetics and behavior, Gilliam says. Meanwhile, zeal to win leads lawyers to overinterpret DNA evidence. It is not always clear that the advances to date in the new field of forensic genetics automatically advance the application of justice.

PHOTO CREDIT: Michael Chu; special effects by Howard Roberts
21stC home 
page 21stC is. . 
. special features next page