Fingerprint Evidence is 'Not Infallible'

15/12/2011 12:51 GMT | Updated 13/02/2012 10:12 GMT

A public enquiry in Scotland has recommended sweeping changes in the use of fingerprint evidence in criminal trials. Fingerprints have long been regarded as the gold standard of forensic evidence, and even a single print can be sufficient to secure a conviction.

But the Scottish inquiry, chaired by Sir Anthony Campbell, is set to change all that. Fingerprint evidence, it concludes, should no longer be treated as infallible. It should, on the contrary, "be recognised as opinion evidence, not fact."

The inquiry concerned the case of Shirley McKie, a successful Scottish policewoman who was accused of leaving her fingerprint at a crime scene and lying about it. In 1997, McKie was part of a police team investigating the vicious murder of Marion Ross in Kilmarnock, Scotland. After the thumbprint of a local builder was found on a gift tag in the victim's home, the man was accused of the murder. When the murdered woman's fingerprints were found on a cookie tin stuffed with banknotes, which McKie discovered when searching the builder's bedroom, it looked like an open-and-shut case.

Then the forensic team discovered something else. They identified a thumbprint on the bathroom door frame at the victim's house as belonging to McKie. This was a serious matter, as McKie had never been granted permission to enter the dead woman's bungalow, which had been sealed off. If she was thought to have crossed the cordon and contaminated vital forensic evidence, she would face disciplinary action. But McKie knew she had never set foot inside the crime scene, so the match between her print and the mark on the bathroom door frame could only be a mistake. Could it have been mislabeled by the fingerprint experts?

The Scottish Criminal Record Office (SCRO) refused even to contemplate the possibility, and McKie was charged with perjury, since she had stated in court that she had never been in the victim's house. Arrested in an early-morning raid, she was brought to the local police station (where her father had been a commanding officer), marched past colleagues and friends, strip-searched, and thrown in a cell. Luckily, two US fingerprint experts came to McKie's rescue. Pat Wertheim and David Grieve spent hours comparing the fingerprint on the door frame with an imprint of McKie's left thumb and concluded that they belonged to different people. Eventually, the jury acquitted McKie of perjury, saving her from a possible eight-year jail sentence.

As she left the court, McKie thought she would receive a formal apology and be invited to return to the job she loved. Instead, she was deemed medically unfit for service and forced into a long legal battle with the police. Although she was eventually awarded £750,000 in compensation, the SCRO did not admit it was wrong, and it is only now that McKie has finally received an apology. "It's the first time I have ever heard anyone say sorry," she told reporters. "This is the first real apology that has been made in 14 years."

Sir Anthony's report highlights the fundamentally subjective nature of fingerprint analysis. When identifying distorted prints, examiners must choose which features to highlight, and even highly trained experts can be swayed by outside information. Yet the subjective elements of this process are rarely pointed out to jurors during court cases. Christophe Champod, an expert in forensic identification at the University of Lausanne in Switzerland, thinks the language of certainty that examiners are forced to use hides the element of subjective judgment from the court. He proposes that fingerprint evidence be presented in probabilistic terms and that examiners should be free to talk about probable or possible matches. In a criminal case, for example, an examiner could testify that there was a 95 percent chance of a match if the defender left the mark. "Once certainty is quantified," says Champod, "it becomes transparent."

Certainty may not seem like the kind of thing that can be quantified, but this is exactly what numerical probabilities are designed to do. By expressing chance in terms of numbers--by saying, for example, that there is a 95 percent chance that a fingerprint was left by a particular suspect--the strength of the evidence becomes much clearer and easier to comprehend. Even with a probability of 95 percent it is clear that there is still a one-in-twenty chance that the mark came from someone else.

Sir Anthony agrees. His report concludes that fingerprint examiners "should discontinue reporting conclusions on identification or exclusion with a claim to 100% certainty". Like science, forensic evidence rarely proves anything conclusively. The unfortunate fact, though, is that most of us simply aren't comfortable with or adept at making judgments in the netherland of uncertainty. Most people have low risk intelligence, and our education system seems particularly bad at developing it. In 1690, the English philosopher John Locke noted that "in the greatest part of our concernments, [God] has afforded us only the twilight, as I may so say, of probability." Yet we are still remarkably ill equipped to operate in this twilight zone.