In June 2002, a corpse turned up near Las Vegas. A fingerprint taken from the body matched one from Kathleen Hatfield of Sonoma county, California. Hatfield鈥檚 mother was notified, funeral arrangements were made and her grave dug. But before the body could be interred, Hatfield turned up alive and well.
Neville Lee was arrested in 1991 in Nottinghamshire, UK, for allegedly raping an 11-year-old girl. The case rested on fingerprint evidence. He was assaulted in jail and his home was wrecked by vigilantes. It was only after another person confessed to the crime that Lee was released.
Last year, an Oregon lawyer named Brandon Mayfield was held in connection with the Madrid bombings after his fingerprint was supposedly found on a bag in the Spanish capital. Only after several weeks did the Spanish police attribute the print to Ouhnane Daoud, an Algerian living in Spain.
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These are just 3 of 22 cases of mismatched fingerprints collected by Simon Cole, a criminologist at the University of California, Irvine (Journal of Criminal Law & Criminology, vol 95, p 985). In many of these cases, people were wrongly convicted and spent time in jail. Yet despite all this evidence to the contrary, fingerprint examiners still say their technique is infallible. What is going on?
Fingerprint matching is undoubtedly a valuable tool for catching criminals but it suffers from one major flaw: nobody knows how often fingerprint examiners make a wrong call. In US federal courts, there are standards that scientific evidence must live up to. One of these 鈥淒aubert criteria鈥� is that techniques must have a known rate of error. Yet, somehow, fingerprint matching has not been required to meet this standard, and the Department of Justice has shied away from systematic research that would reveal the error rate (New 杏吧原创, 31 January 2004, p 6).
US fingerprint examiners make a distinction between 鈥渕ethodological error鈥� and 鈥減ractitioner error鈥�. The first enshrines the notion that if an examination is done correctly the error will always be zero. Practitioner error is an admission that people can get things wrong, but FBI examiners argue that this error should be excluded from the Daubert criteria.
It is a seminal idea in science that all errors should be understood and quantified, whether they come from the method used, the equipment or human errors. But ultimately, it is the overall error that counts. Splitting error in two, as examiners do, may be useful for calculating overall risk, but applying Daubert criteria to only one of them makes no sense.
Take automated fingerprint recognition, which is being adapted for use in passports, as a digital identity for banking and even as an alternative to car keys. Just deciding which parts of the hardware and software play the role of the methodology and which the practitioner would tax a philosopher. But all that really counts is the overall error rate: how often do immigration officials fail to recognise you, how often are you prevented from withdrawing cash or locked out of your car.
鈥淚nnocent people are being wrongly convicted, and the numbers could be substantial鈥�
The error rates of these automated systems are well understood, and are likely to cause plenty of problems for us all as the use of biometrics spreads (see 鈥淧rivacy and prejudice: whose ID is it anyway鈥�). Understanding the error rates in judicial fingerprint matching will also create problems: any criminal whose conviction relied heavily on fingerprint evidence is likely to appeal. But that is no reason to ignore the issue.
Innocent people are being wrongly convicted, and the numbers could be substantial. One disturbing aspect of Cole鈥檚 list is that two-thirds of the mistakes emerged only in exceptional circumstances, such as an unexpected confession. How many other errors have failed to come to light from mundane cases?
Ignoring the existence of error also prevents fingerprint analysis being improved. Examiners鈥� decisions are clearly influenced by what they are told before they examine a potential match (see 鈥淓xaminers鈥� objectivity called into question鈥�). Such biases will remain so long as examiners refuse to acknowledge them.
As more mistakes are exposed and defence lawyers intensify their challenges, it is only a matter of time before judges and juries reject fingerprint evidence. That would be a dreadful waste of a powerful tool. Far better to do some research now so we know once and for all how confident we can be about fingerprints.