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THE MYTH OF FINGERPRINTS

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Page 1Page 2Page 3Page 4Page 5Page 6Page 7Page 8Lingua Franca 2000 10(8) 54-62The Myth of FingerprintsA forensic science stands trialBy Simon ColeA COURTROOM LOOKS DIFFERENT WHEN YOU ARE SITTING ON THE WITNESSSTAND. The imposing trappings of a medieval institution; the small crowd of silent onlookers;the elation that comes of debating issues one has long considered in isolation, tempered by theclaustrophobia of the interrogation and the naked deployment of authority – it all seemed soreminiscent of...well, of my doctoral defense. When I first chose to write my dissertation on how fingerprint examiners achieve credibility asexpert witnesses, I never thought that I would end up an expert witness myself. But there I sat inthe witness box, fidgeting nervously, swearing to tell the truth. Here was a role reversal worthy ofthese postmodern times, one that brought new meaning to the term "participant observation." But for Byron Mitchell, a quiet, husky, bespectacled bear of a man in his mid-fifties, theproceedings were anything but academic. Mitchell was on trial for allegedly driving the getawaycar in the 1991 robbery of an armored vehicle. No one had been hurt, but Mitchell, facing aconviction for armed robbery, was looking at spending most of the rest of his life in prison. Theforensic evidence that linked him to the getaway car was the toughest sort to beat: twofingerprints lifted from the steering wheel and the gearshift. Mitchell's only hope was toundermine the credibility of the world's most widely trusted form of forensic proof. It was thisendeavor that brought him to a federal courtroom in Philadelphia on a sweltering July morning,where he sat surrounded by four earnest public defenders, a determined prosecutor, some of thecountry's leading fingerprint examiners, three iconoclastic academics, and a patient but skepticaljudge. Most laypersons regard fingerprint identification as the epitome of science in the service of thelaw – indeed, as the model to which all other forensic sciences aspire. And yet this isn't quite so.Fingerprint identification puts the legal definition of science, recently articulated in the landmark1993 Supreme Court decision Daubert v. Merrell Dow, to one of its most difficult tests."Fingerprint evidence," Arizona State University law professor Michael Saks asserted in 1998,"may present courts applying Daubert with their most extreme dilemma.... A vote to admitfingerprints is a rejection of conventional science as the criterion for admission. A vote forscience is a vote to exclude fingerprint expert opinions." The authority of fingerprint evidence rests on two contested assumptions. Although conventionalwisdom since the nineteenth century has accepted the doctrine that no two fingerprints are alike,no one has really proven the proposition's validity. But if the question of the uniqueness offingerprints seems pedantic, consider a more practical concern: How reliable is fingerprintevidence anyway? Can forensic technicians really match a fragmentary or smudged print takenfrom a crime scene to one and only one human fingertip, to the exclusion of all others in theworld? At a pretrial hearing in the Mitchell case, this important question would receive its firstairing in a U.S. courtroom.FOR HUNDREDS OF YEARS, the uniqueness of fingerprint patterns has been so widelyassumed that it never seemed necessary to prove it. As far back as the fourteenth century, thePersian historian Rashid ad-Din, reporting the use of fingerprints as signatures in China, declaredthat "experience shows that no two individuals have fingers precisely alike." At the beginning ofthe modern era of fingerprinting, John S. Billings, a U.S. Army physician, declared, "Just as eachindividual is in some respects peculiar and unique, so... even the minute ridges and furrows at theend of his forefingers differ from that of all other forefingers and is [sic] sufficient to identify." The first person to fashion a statistical foundation for this assumption was the British gentlemanscientist Sir Francis Galton. He calculated the probability that any two fingerprints wouldresemble each other in all particulars as one in sixty-four billion. Galton also noticed that thepapillary ridges on the fingertip often end abruptly or split and rejoin one another. In his 1892book Finger Prints, Galton labeled these points at which ridges end or split "minutiae"; theywould later be known as "ridge characteristics," "points of similarity," and "Galton details."There are about thirty-five to fifty such points on a typical finger. By correlating the minutiaewith one another, an expert could match two fingerprints. Inspector Charles Collins of Scotland Yard put Galton's observations to work in the famous 1905Deptford murder trial, in which the brothers Albert and Alfred Stratton stood accused ofmurdering a shopkeeper and his wife in a London suburb. Collins compared Alfred Stratton'sinked fingerprint with a bloody fingerprint found on a cash box and concluded that there wereeleven matching "characteristics." The inspector's testimony convinced the jury but not the judge,who, bound by the jury's verdict, "almost apologetically" sentenced the Stratton brothers to hang.How secure was Collins's contention that he had matched the print to Alfred Stratton's finger, tothe exclusion of all others? And how many matching minutiae did Collins require in order toreach his conclusion? No one really knew. Oddly, the leading critic of Collins's methods was a Scottish physician named Henry Faulds – thevery man who had first suggested to Galton (via Galton's cousin, Charles Darwin) that "greasyfinger marks" might be used to link criminals to their crimes. In a pamphlet published just afterthe trial, Faulds complained that "the popular fiction, that no two fingers can be alike," was beingtreated as "a sober fact of the highest scientific certainty." And yet, he pointed out, "the onlyproof of it is seemingly the same 'fact' repeated in other words – that Scotland Yard by its systemof classification has never been able to find two fingers alike." The absence of disproof, in otherwords, was being treated as proof. Moreover, based on his own experience studying fingerprints,Faulds claimed that "'repeat patterns' in single fingers are often found which come so near, theone to the other, that the least smudginess in the printing of them might easily veil importantdivergences with appalling results." Why had Faulds gone from early


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