Forensic advances can play a big part in like Simpson's
by Shannon Brownlee, Monika Guttman, Ted Gest, David Bowermaster, Dorian Friedman
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Though the list of evidence gathered by authorities against O.J. Simpson continues to grow, his fate will almost certainly hinge on a piece of evidence that did not appear in the courtroom last week -- the contents of his genetic material. It is likely that the core of the prosecution's case -- or the basis of Simpson's defense, depending on the results -- will rest on a revolutionary and controversial forensic technique known as genetic profiling, or "DNA fingerprinting."
In theory, every human being other than an identical twin can be identified by his or her unique genetic code; DNA extracted from blood or other tissue at a crime scene can be compared with a suspect's genetic material. In practice, however, DNA profiling has been hotly debated ever since it was first introduced in a courtroom nearly a decade ago. Proponents, including the Federal Bureau of Investigation, argue that the likelihood that samples of DNA from two different persons will appear to match is minuscule. They commonly cite odds as low as 1 in several million. But critics charge that prosecutors and the FBI claim more accuracy for these techniques than they warrant -- and that they inflate probability figures to downplay the chances of a mismatch.
Those are the arguments that will play out in this case. Last week, Simpson's attorney Robert Shapiro called in New York lawyers Barry Scheck and Peter Neufeld, the nation's leading experts in defending against DNA evidence, to be part of the defense team.
When DNA profiling first was used in a U.S. court in 1987, it was hailed by forensics experts as the greatest advance since fingerprinting. The most accurate method of DNA profiling, involving restriction fragment length polymorphism, or RFLP, requires extracting genetic material from small amounts of body tissue or fluid -- blood, saliva, bone, skin or a hair follicle. A bloodstain as small as a quarter will do. Tiny fragments of the DNA are then used to generate images that look something like supermarket bar codes, which are examined visually for a match with DNA fragments from the suspect. RFLP profiling, which takes weeks to complete and can cost several thousand dollars per sample, has been used to link at least 700 individuals to crimes.
Exoneration, too. There is no argument that DNA patterns from a crime scene that do not match a suspect's can lead to speedy exoneration. In January, Mark Diaz Bravo was released from Tehachapi state prison in California after spending three years behind bars for the rape of a mental patient: DNA profiling showed he could not have been the source of semen found at the scene of the crime. At least 15 men have been freed after serving time when comparisons of their DNA failed to match evidence gathered at crime scenes. Neufeld and Scheck now represent more than 200 other convicted felons who believe DNA tests will prove their innocence. If DNA tests in the Simpson case fail to make a match, says one expert in DNA evidence, "the prosecution will be blown out of the water."
DNA profiling gets controversial only when a match is made. Like a fingerprint, a person's entire genetic code is unique. But genetic profiling examines just a tiny fraction of a person's DNA. To demonstrate that two DNA samples match because they come from the same person and not purely by chance, proponents use statistical models based on samples from the population at large. Critics, including some scientists, call these statistics bad science. They claim that prosecutors have exaggerated the rarity of various DNA profiles, raising the possibility that DNA evidence could falsely implicate the accused. They also charge that labs performing the delicate analysis are not subject to adequate quality controls.
Until recently, few defense attorneys could refute the prosecutions' complex calculations. That left juries in awe of the apparent accuracy of DNA tests. Most trials where genetic evidence is admitted end in conviction, and in only a handful of cases has the jury chosen to ignore DNA. The power of genetics was evident last April when Timothy Spencer was executed in Virginia -- the first person in America put to death on the basis of a DNA "fingerprint."
In recent years, though, defense attorneys have grown more successful in attacking DNA profiling. The debate has left appellate courts confused. In most states, scientific evidence is admissible only when it has "gained general acceptance" in the scientific community. That standard was recently overturned by the U.S. Supreme Court, giving judges more latitude in accepting such evidence. California, however, still abides by the old, restrictive standard, and at least one state appeals court has ruled DNA evidence inadmissible. One prominent California criminal defense attorney says that if DNA tests do not exonerate Simpson, "the trial will come down to whether or not the evidence is admissible."
Most legal observers predict that it will be. The result, says Peter Keane, chief attorney in the San Francisco public defender's office, will be a protracted scientific battle: "One regiment of scientists will come in for the prosecution saying DNA is the greatest thing since sliced bread, and another regiment will come in for the defense and say it's all smoke and mirrors."
Playing it safe. The prosecution will likely play it safe by presenting odds of a mismatch that are based on the most conservative statistics. Standard calculations often produce odds as low as 1 in a million that DNA from two different sources could result in a match, according to Mark Stolorow, director of operations at Cellmark Diagnostics, the company that often does DNA tests for the Los Angeles Police Department. The more conservative method, recommended by a panel of scientists two years ago, might bring the odds down to 1 in a few thousand. Even so, says David Kaye, an expert in scientific evidence at Arizona State University in Tempe, "Once the judge agrees that statistics can come in, the defense will have a hard time of it." Simpson's team will be left to argue that the lab bungled the tests, or that the presence of Simpson's blood at the scene of the crime was a tragic coincidence.
The DNA tests are not expected for several weeks, but other forensic evidence already raises problems for Simpson. Tests for proteins found in blood at the scene, as well as the results of a faster though less-precise DNA test called PCR, reportedly match Simpson's blood. Of the 113 pieces of physical evidence, some no more substantial than a few strands of blond hair found on a brown leather glove at Simpson's home, only a fraction will be subjected to the more lengthy RFLP testing. Genetic tests may help determine whether or not blood found in Simpson's car and at his home could have come from the victims.
Whatever the outcome of this celebrated trial, genetic profiling is almost certainly here to stay. Within the decade, the faster PCR genetic test, coupled with more discriminating techniques, will require even less tissue and produce far more certain odds of a match. Though DNA testing has been used in only a small fraction of rape and murder cases, juries are beginning to expect genetic evidence. Paul Ferrara, director of the Virginia State Division of Forensic Sciences, the first state lab to perform DNA analysis, says, "It's difficult when you have a technique as powerful as DNA for the courts to accept anything less. Juries are saying, `Where's the DNA?'"
