Is Fingerprint Evidence Sound Science?

We carry around unique maps of swirls and ridges on our fingertips, and judges and juries have been using them for almost a century to decide innocence and guilt. Because no two people have the same patterns, fingerprint evidence has been nearly irrefutable in courtrooms for decades — until now.

A federal judge in Philadelphia has become the first to question the scientific soundness of fingerprint evidence.

U.S. District Court Judge Louis H. Pollak last week ruled that such evidence does not meet standards of scientific scrutiny established by the U.S. Supreme Court, and said fingerprint examiners cannot testify at trial that a suspect's fingerprints "match" those found at a crime scene.

The interpretation of fingerprint evidence has already been the source of heated debate in the forensic community, and Pollak's decision, coming after two years of legal challenges around the country, is likely to stoke the controversy. At issue is not whether a fingerprint is a unique identity marker, but whether fingerprint experts have scientifically sound techniques in determining identity from even a partial smudge of a fingerprint found at a crime scene.

Ruling Could Spawn Similar Cases, Decisions

Pollak's decision only applies to a murder trial scheduled to start later this month in Philadelphia — and shouldn't affect past convictions based on fingerprint evidence — but experts say it likely will have broad resonance. The ruling could serve as a basis for similar decisions in other courts and prompt further legal challenges based on its reasoning.

In the Philadelphia case, lawyers for three men accused of running a drug ring associated with four murders sought to bar fingerprint evidence from their upcoming trial. Pollak allowed fingerprint evidence to be presented to the jury but will not permit experts to testify that the crime scene prints "match" those of the defendants.

There is no "scientific testing that tended to establish the reliability of fingerprint identifications," Pollak ruled. However, Pollak did not reject the overall use of fingerprint evidence, saying witnesses could discuss similarities and differences between fingerprints and that the imprints are "permanent" and "unique."

In his ruling, Pollak relied on a 1993 U.S. Supreme Court decision that required judges to take a more direct role in deciding what scientific evidence could be permitted at trial. The court established the "Daubert" guidelines, which are designed to prompt questions about the scientific sturdiness of certain types of evidence, such as whether it has been adequately tested, what its error rate is, and, in the case of fingerprinting, whether there are standards for what constitutes a "match."

Handwriting, Hair Evidence Also Questioned

Fingerprints have been used in courtrooms since the early 20th century, and judges have generally regarded them as scientifically sound. In reality, says evidence expert Jennifer Mnookin, the use of fingerprinting has never withstood rigorous scientific testing standards.

"Instead of taking it on faith, [Pollak] decided to examine whether the claims of fingerprint examiners have been sufficiently tested," said Mnookin, a professor at the University of Virginia School of Law. "The testing under Daubert is supposed to be testing by the scientific community, not testing through the adversarial process in court."

Other kinds of forensic evidence, such as handwriting and hair samples, have come under similar scrutiny in recent years. For the last two years, about a dozen cases have circulated in the court system challenging the use of fingerprint evidence using the Daubert test.

But until now, no judge has ever questioned the reliability of fingerprint evidence as Pollak has.

"If you had asked a forensic scientist a few years ago what is the 'citadel,' they would have said, whatever else happens, courts won't touch fingerprinting. That's gospel," says Edward Imwinkelried, professor of law at University of California, San Diego. "Now, Pollak says if you want to call yourself a scientist in court, you'll have to pass muster. Nothing is sacrosanct."

Does Ruling Create Chaos?

That Pollak, a former dean of the Yale Law School and well-respected senior federal judge, was the first to question fingerprint evidence is significant, some experts said.

"I think a ruling from a judge of his stature will carry a lot of weight," said Simon A. Cole, author of Suspect Identities: A History of Fingerprinting and Criminal Identification. "It's a significant ruling. There are now judges on two sides of the issue, and other judges will be able to pick one."

There are indeed two sides to the issue, and some forensic experts are questioning the soundness of Pollak's ruling and defending fingerprint evidence as a reliable forensic tool.

Allowing the jury to see fingerprint evidence but prohibiting expert witnesses from interpreting the evidence only invites chaos, says Andre A. Moenssens, Douglas Stripp Professor of Law at the University of Missouri at Kansas City.

Moenssens, a former certified latent print examiner who has worked in the field almost half a century, says he sees Pollak's decision as a legal aberration.

"The last couple of years, all other courts have held [fingerprint evidence] is reliable and the testimony is admissible," he said.

But other experts see Pollak's ruling as a foray into a new era of re-examining a time-worn tool of crimefighting.

Other judges may decide to hold fingerprinting to the same standard, and forensic scientists may use the ruling as an opportunity to launch more vigorous studies of fingerprinting.

"This could create an incentive for the fundamental research needed to validate the underlying premises of the technique," says Imwinkelried. "That's the salutary effect of a decision like Daubert."