For decades, fingerprinting and handwriting analysis have enjoyed sure acceptance in American courts.
But now, in an age of scientifically grounded DNA analysis, judges are looking with increased skepticism at forensic techniques that rely solely on the subjective experience of experts.
Last week, Philadelphia federal judge Louis Pollak ruled a fingerprint expert could not testify that a suspect's prints definitively matched those found at a crime scene. Other courts have also recently knocked out testimony from handwriting and knife-mark experts.
Legal observers view the decision by Judge Pollak as a breakthrough that could lead to major changes in the way everything from firearms to bite marks are investigated and presented in court.
"Courts are forcing forensic science to become a science - to actually test its claims, determine its error rates, and not overstate conclusions," says Michael Saks, a law professor at Arizona State University in Tempe.
Both fingerprinting and handwriting analysis were developed by police in the early 1900s, based on the premise that each person's unique prints or penmanship assures that the right suspects are tied to their crimes.
But fingerprint experts and their critics have disagreed over how well a partial print lifted from a crime scene can be matched with certainty to a full print culled from a database.
Still, until a decade ago, federal judges were limited in their discretion to bar an expert's testimony. To make such a determination, the judges could only ask whether the testimony was accepted within the community of experts in that field.
But then, in 1993, the Supreme Court gave federal judges the power to decide for themselves whether an expert's testimony was sufficiently grounded in science to be admitted. In effect, this made federal judges the gatekeepers over all scientific and forensic evidence.
In recent years, defense attorneys have launched at least a dozen challenges to fingerprinting analysis. Before last week, though, none had succeeded.
In his decision, however, Judge Pollak found that although fingerprint science had been accepted in court for 90 years, it hadn't been proved scientifically. He ruled that a fingerprint expert could still testify - but could not tell jurors as a matter of scientifically proven opinion that the defendant's fingerprint was a match to one at the crime scene.
Furthermore, last August in Alaska, federal judge H. Russel Holland barred testimony regarding handwriting.
Patrick Meehan, US attorney for the Eastern District of Pennsylvania, says that blocking conclusions about forensic evidence could make it more difficult to build a persuasive case. "It certainly will make a prosecutor's job harder," says Mr. Meehan, who indicated his office will appeal the decision.
But defense attorneys say raising the bar on expert forensic testimony prevents an aura of truth from seeping into cases where it doesn't belong. "If an expert testifies with all his credentials, it has the effect of a confession" on a jury, says Kevin McCoy, an assistant federal public defender who challenged handwriting analysis in the Alaska case last year.
In the past decade, forensic tools that rely on the interpretation of experts have suffered in comparison with DNA analysis, which has more proficiency tests and accredited labs, says Lawrence Kobilinsky, a professor at the John Jay College of Criminal Justice in New York. "None of them have gone through the same scrutiny and validation," he says.
In fact, in cases where convicted defendants are exonerated using DNA evidence, studies show that erroneous or fraudulent forensic testimony is the second-most common cause of error at trial, according to Mr. Saks of Arizona State.
For now, fingerprint examiners say the latest decision will not make them change how they conduct investigations, but it will push them to highlight the science underlying their work.
"The average fingerprint examiner is taught to do something, and they gloss over how they got to that decision," says William Leo, a forensic identification specialist with the Los Angeles County Sheriff's Department. "We're going to do a lot more training of people about how to explain the scientific research regarding the decisions they make."