News your connection to The Boston Globe

A blow to the credibility of fingerprint evidence

SEVEN YEARS ago a fingerprint left on a glass of water sent Roxbury resident Stephan Cowans to prison for 30-45 years for shooting a police officer.


Cowans was recently released after startling revelations made it clear that he was not the shooter. DNA tests on clothing left near the crime scene and on a saliva specimen from the glass did not match Cowans's DNA. The prosecution still insisted it had the right guy -- after all, his fingerprint was on that glass. But when that fingerprint was reanalyzed by experts, it turned out not to match Cowans after all.

However this particular fingerprint error ends up being explained, it raises important issues that go well beyond the Boston Police Department. The science of fingerprints, once thought to be impregnable, is far from secure. Courts must be hesitant to admit fingerprints as evidence until there is better information about how often mistaken identifications are made.

There are only three possible explanations. One is fraud on the part of the fingerprint examiner. Another is incompetence. The third is error. Each reveals a significant problem in our current use of fingerprint evidence.

What if it's fraud? While most forensic scientists are dedicated professionals, the fact that forensic science is conducted in state crime labs and police departments creates implicit pressures to please the prosecution. Moreover, forensic investigators often know about other evidence linking a suspect to a crime, and this outside knowledge may color their forensic analysis. Independent crime laboratories could insulate forensic experts from both bias and the temptation to be too cozy with the prosecution.

In this case, the district attorney has suggested that the error was probably just an honest mistake, as if this ought to provide some comfort. But it shouldn't. At present, there is virtually no accurate information on just how often fingerprint examiners actually make mistakes -- and unlike DNA experts, fingerprint experts routinely testify that their matches are "100 percent certain."

What is troubling is not that such mistakes might happen. No forensic technique could ever be error-free. But the present dearth of information on mistakes -- either from incompetence or "honest" error -- is simply inexcusable. Fingerprinting is often said to be be infallible, a forensic "gold standard." But if we ask how often declared fingerprint matches are actually wrong, the only honest answer is that no one has any idea.

There are no systematic proficiency tests to evaluate examiners' skill. Those tests that exist are not routinely used and are substandard. In another recent case, even the FBI's proficiency tests were acknowledged by another fingerprint examiner to be absurdly easy. While challenging proficiency tests would not be perfect, they would provide significantly more information about error rates.

More shocking, there are no uniform standards, locally or nationally, about what counts as a fingerprint match. Different jurisdictions, and even different examiners, have different criteria, and the courts have simply left it to the experts' judgment.

In addition, we have no idea how often two individuals -- whose prints would indeed look different if we had access to a complete set of 10 undistorted prints -- might have partial fingerprints that resemble each other enough for an examiner reasonably to mistake them as coming from the same person, especially when the print lifted from the crime scene might be smudged and distorted.

Fingerprints are clearly an enormously valuable investigative tool. But the fingerprint community has little motivation to investigate how often they make mistakes. Fingerprint examiners regularly assert in court that the technique is error-free and that fingerprint matches are a sure thing. Whatever real research ends up showing, fingerprints cannot possibly be as perfect a technique as the experts presently claim. Unless courts make better knowledge about error rates a precondition of its use as legal evidence, the fingerprint experts will have no incentive to measure them.

Fingerprint evidence has enormous cultural power -- in Cowans's case, the prosecutor had said he was prepared to prosecute again, despite the exculpatory DNA findings, precisely because of that supposed fingerprint match. Although numerous defendants have challenged the use of fingerprint evidence in court in the past few years, judges for the most part have not taken these challenges as seriously as they should. Whatever happened in this case, it should be a wakeup call to experts, prosecutors, judges, and the public. Until the limits of fingerprint evidence are better understood, we must be wary.

Fingerprints may be unique. This error, almost certainly, is not.

Jennifer L. Mnookin is visiting professor of law at Harvard Law School.

Globe Archives
Today (free)
Yesterday (free)
Past 30 days
Last 12 months