Courts have historically been impressed with the legal testimony of experts simply because of their credentials.
But legal and scientific forces are now skeptically discussing the claims of traditional forensic sciences, according to Koehler. One reason for that is the effectiveness of DNA testing, which Koehler notes has endured rigorous scrutiny from a broad range of scientists and legal scholars.
While DNA fingerprinting is based on well-tested genetic assumptions, traditional methods generally rest on the assumption of "discernable uniqueness." This is a common supposition that two apparently identical-looking marks were produced by the same object. (See sidebar.)
"The problem with this assumption is that there is no way to verify it," Koehler said.
"If forensic scientists find no unexplained differences between [two marks], they infer that the marks were made by the same person or object," he said. "But how do we know this to be so?"
Forensic errors are common, despite industry claims to the contrary, according to the study.
Voice identification error rates are as high as 63 percent, depending on the type of voice sample tested. Handwriting error rates average around 40 percent, and false-positive error rates for bite-marks run as high as 64 percent.
Fingerprint examiners may fare better. But data from one forensic testing program show that, since 1995, about one-fourth of examiners have failed to correctly identify all latent prints in the test. In one test, 20 percent of the examiners mistook one person's prints for those of his twin.
During the past decade, scores of people who were convicted of serious crimes have been exonerated by DNA analyses of crime-scene evidence that had not been tested at the time of their trials.
Drawing on data from 86 such cases, Koehler and his colleague, Michael Saks, a law professor at Arizona State University in Tempe, found that forensic science testing errors played a part in 63 percent of the wrongful convictions. Only eyewitness error was a more common factor.
The study also found that forensic scientists are the witnesses most likely to present misleading or fraudulent testimony. In 27 percent of the cases reviewed, expert witnesses were found to have given false testimony.
"We think the results are surprising, because the public tends to think of forensic science evidence as pretty close to infallible," Koehler said.
"One problem is that the public confuses the theoretical power of a technique with the practical realities of using it correctly under less-than-ideal conditions," he said.
Koehler speculates that the close relationship between police, prosecutors, and forensic scientists often creates pressure on forensic scientists to be something other than a dispassionate provider of scientific information.
U.S. Supreme Court decisions over the last decade have required courts to evaluate the scientific merit of proffered scientific evidence.
Experts say the field of DNA typing, with its reliance on statistics and extensive databases, can serve as a model for the future of traditional forensic science.
"It is not, however, as simple as that," said Siegel, the forensic science program director. "DNA types are finite. There are only so many DNA types that exist. The same may not be said for identification evidence."
Handwriting characteristics, for example, may lie on a continuum with no set number of "types." The number of ways that fingerprint ridges may be arranged is very large, if not infinite, and is not easily quantified. The same may be true with bullet and tool-mark striations.
"That's not to say efforts shouldn't be made to put these evidence types on a more scientific footing," Siegel said.
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