Printing errors

“BOOK him”, says the crusty sergeant at the local precinct, and another villain is led off to be fingerprinted. For nearly a century, suspects have lived in fear that matches to their prints will turn up at the scene of the crime. Such fears have now been reduced a little. In a ruling on January 7th, Louis Pollak, a federal judge in Pennsylvania, decided that fingerprint evidence was unreliable. He declared that he will no longer allow fingerprint examiners testifying in his courtroom to tell juries point blank whether prints from defendants and those collected at crime scenes do or do not match. Instead, they will have to present evidence to persuade a jury that they are the same or, as the case may be, are not.

This is the first ruling of its kind in the American courts, although fingerprinting evidence has been open to such a challenge for years. In the 1990s America's Supreme Court deemed it the responsibility of federal judges to insist that expert witnesses testify about the reliability of a forensic-scientific method only if the method in question has been tested so that the range of its error rate is known. Fingerprinting experts have long claimed that their error rate in matching prints is zero—but without any supporting evidence.

This claim is scientifically dubious, for two reasons. One is that many prints collected at crime scenes are so-called latent prints, meaning that it takes special chemical treatments, or illumination with ultraviolet light, to “recover” them. Such latent prints are often incomplete and indistinct, and might not produce unique matches. The other reason is that declaring a match between two prints generally requires a certain number of points of similarity between them. Different jurisdictions set varying standards for how many similar points are required, which makes this standard seem arbitrary. Even worse, in some jurisdictions declaring a match requires only an overall “impression” of similarity on the part of an expert.

Judge Pollak's 49-page ruling leaves plenty of room for debate. The judge did not dismiss fingerprint evidence entirely, saying that such a step would be “unwarrantably heavy-handed”. He stated that witnesses for both sides in United States v Plaza could testify how fingerprints were obtained and about similarities or differences between them. That will allow defence lawyers to poke holes in the testimony of the prosecution's witnesses. The judge will also permit experts to tell juries the oft-repeated dogma that every person's fingerprints are unique, a claim that the defence will no doubt challenge.

Challenges to fingerprint evidence—not to mention other dodgy forensic methods—will keep growing

The controversy will continue outside Judge Pollak's courtroom, too. Other American courts are not necessarily bound by the ruling, and America's Department of Justice may seek to reverse it. Still, Judge Pollak's views carry considerable weight because he is a former dean of both the Yale and the University of Pennsylvania law schools, and has repeatedly been invited to sit on the appeals courts. Two professors of law—Elizabeth Phillips Marsh of Quinnipac University in Connecticut, who heads the American Bar Association's section on evidence, and Jennifer Mnookin of the University of Virginia—have called his ruling “exceptionally well-reasoned”.

Regardless of how Judge Pollak's ruling fares, two changes are likely. One is that challenges to fingerprint evidence—now more than 20 in America alone—will keep growing, especially in cases where other evidence suggestive of a defendant's guilt is weak or missing. The other is that, if fingerprints lose their scientific status, so might other dodgy forensic evidence, such as handwriting analysis or making matches from eye and hair characteristics. Ballistics experts might also come under fire, because the marks left on bullets change as the rifling inside a gun barrel undergoes the wear and tear of repeated use.

Even the current “gold standard” of evidence, DNA testing, may face challenges. However, matches made with DNA are already expressed as statistical probabilities rather than as certainties. Thus it is the admitted error of DNA testing, rather than its claims to infallibility, that will safeguard its continued use in the courts.