A single fingerprint found at the scene of a crime is such powerful evidence that it's almost an automatic conviction. Fingerprints never lie: Juries have been told that for more than a century.
But a criminologist at the University of California, Irvine, has documented 22 cases, most involving violent crimes, in which fingerprint evidence turned out to be dead wrong, usually discovered after defendants had served time for crimes they did not commit.
The fingerprints didn't lie. But the experts who matched them with a suspect were wrong, and subsequently had to admit it, according to court records analyzed by Simon Cole, assistant professor of criminology, law and society at Irvine.
Cole's exhaustive research argues that the "zero error rate" claimed by fingerprint experts needs serious retooling. His findings are published in the current issue of the Journal of Criminal Law & Criminology.
The cases include one man who volunteered for hazardous assignments while in prison to earn money to pay for DNA research involving his case. That research eventually cleared him, despite the fact that experts had testified that his fingerprint implicated him in the attempted murder of a police officer.
In another case, a corpse found in the Nevada desert was identified – on the basis of fingerprints – as that of a California woman who later turned up alive.
Most of the cases studied by Cole were covered by the media, partly because they involved sensational events like the Madrid train bombing last year that killed 191 people. Cole suspects there are many other cases, possibly more than 1,000 each year in the United States alone, in which fingerprints have been matched erroneously with the wrong person.
Cole isn't saying that fingerprints are not a useful tool for law enforcement. All he's saying is their "error rate" is more than zero, and some way needs to be found to put their reliability into perspective.
"To tell the jury that it's positive identification, that it's infallible, that it's 100 percent certain is overstating the value of it," Cole said in an interview. "It may well be that it's right 95 percent of the time, but when it's wrong it's very unlikely for us to know about it. So it's likely to result in a miscarriage of justice."
There are multi-layered programs designed to prevent errors, including a requirement for independent verification. Cole found only 22 cases, spanning several decades, but he was limited to cases that have come to public attention.
"An analysis of these cases shows that they are most likely only the tip of the proverbial iceberg of actual cases of fingerprint misattribution," he writes.
In most of the cases he studied, the error did not surface during the routine processing of the case. It surfaced because of "extraordinary circumstances," such as the confession of someone else, which Cole describes as "always a fortuitous and highly unlikely event." So he concludes there are probably many less "fortuitous" cases where the error is never discovered.
Part of the problem stems from the nature of the evidence itself. Fingerprints taken from the scene of a crime (called latent prints) are not always clearly defined.
"They are typically partial, smudged or otherwise distorted," according to Cole.