Experts are often called upon to testify that a criminal defendant’s fingerprints match a fingerprint found at the crime scene. Combined with other evidence of a defendant’s guilt, a fingerprint can be convincing proof of criminal responsibility. But when a case hinges entirely upon the unexplained presence of a defendant’s fingerprint, guilt may depend on the presumption that no other person could have left that print.
The uniqueness of fingerprints is increasingly being called into question. Scholars disagree as to whether forensic fingerprint identification has been scientifically validated. As a practical matter, the only way to be certain that fingerprints are unique would be to compare the fingerprints of every person who has ever lived. That obviously can’t be done.
Even if fingerprints really are unique, the fallible nature of fingerprint identification is illustrated by the wrongful conviction of Stephen Cowens. After Cowens spent more than six years in prison, Boston police and prosecutors admitted they were mistaken when they said that fingerprints found at a murder scene “unquestionably” belonged to Cowens. Several other wrongful convictions have hinged on the testimony of fingerprint experts who made incorrect identifications.
The highly publicized arrest of Brandon Mayfield for terrorism was based on the FBI’s mistaken identification of a fingerprint. According to the FBI, it used “standard protocols and methodologies,” including verification by outside experts, to link the suspect’s print to Mayfield. The FBI eventually conceded its error, explaining that it relied on a digital image of the suspect’s print that was of “substandard quality” and that there were a “remarkable number of points of similarity” between Mayfield’s prints and the suspect print.
Questionable Identification Techniques
Forensic experts might be mistaken when making a fingerprint identification for a number of reasons. Fingerprints within families tend to be very similar. As people age, skin elasticity changes, which alters fingerprint impressions. Some skin conditions also affect fingerprints. Even two prints left by the same finger are never identical. Skin stretches. The angle at which the finger touches a surface, and the pressure exerted, will affect the fingerprint impression.
Prints recovered from a crime scene may be smudged or distorted. Sometimes only a partial print can be recovered, making identification problematic. Too often, the fingerprint analyst knows that the police suspect the guilt of a particular person, and that knowledge may instill an unconscious bias that blind testing (a key component of the scientific method) would remove.
Fingerprint examiners compare “points of similarity” between a fingerprint recovered from a crime scene (a “latent print”) and a suspect’s fingerprint. When the fingerprints are clearly dissimilar, the suspect should be excluded as the source of the print. When there are enough “points of similarity,” they typically testify that the suspect is the source of the latent print. In some cases, experts testify that the suspect is the only person in the universe who could have left the latent fingerprint.
Professor Simon Cole persuasively argues that the boundary between sufficient and insufficient “points of similarity” to make a definitive match has never been scientifically established. In an influential article, Cole examines 22 cases in which reputable fingerprint experts reached a “consensus judgment” that a latent print identification, relied upon as proof of guilt in a criminal case, was incorrect. In some of those cases, the defense used its own fingerprint expert to challenge the conclusion drawn by the prosecution’s expert. Some of the defendants were acquitted or their cases were dismissed, while others were wrongly convicted. In several cases, the prosecution’s expert testified that 12 to 14 points of similarity proved that the latent fingerprint belonged to the defendant.
Cole suggests that the 22 cases are “the tip of the iceberg.” He also suggests that the prevailing Daubert standard of expert evidence admissibility fails to act as a shield against unreliable fingerprint analysis because courts are insufficiently skeptical of claims that fingerprint identification is infallible.
How Should Fingerprint Experts Testify?
For many years, fingerprint experts testified that fingerprint identification is infallible. That view was supported by the FBI’s publication, The Science of Fingerprints: Classification and Uses. While fingerprint identification may be reliable in most cases, it is now clear that it is not infallible.
Simon Cole points out that experts have repeatedly testified that the “error rate” in fingerprint identification is zero. As the cases of Mayfield and Cowen (among others) demonstrate, that testimony is untrue. Cole also points out that fingerprint analysts have been encouraged to testify that the methodological error rate is zero while the practitioner error rate is unknowable but negligible. In other words, if identification methods are followed precisely, the error rate is zero, but there is no way to know how often analysts fail to follow the methodology.
Since the methodology does not exist in a vacuum but is always applied by a practitioner, Cole argues that any attempt to draw a distinction between the two error rates is specious. He suggests that ethical fingerprint analysts should never testify about a methodological error rate of zero, because that testimony is both meaningless and misleading.
In any event, many experts now agree that the “zero error rate” claim is not scientifically supportable. Since there is no way to be certain that every person has a unique set of fingerprints, even a perfect match, determined by state-of-the-art techniques, cannot be said with certainty to be error-free. In any event, the “zero error rate” claim is misleading because it invites the jury to believe in the mythical perfection of fingerprint science while deflecting attention from all the real-world factors that result in misidentifications.
Mike Silverman, the fingerprint expert who introduced an automated fingerprint detection system to London’s Metropolitan Police, thinks experts have a duty to make juries aware of the problems with fingerprint analysis. Silverman worries that jurors are influenced by shows like CSI. According to Silverman, the certainty that television viewers see on CSI “simply doesn’t exist” in the real world.
If a fingerprint expert determines that a latent print matches a suspect, it may be unlikely that the fingerprint belongs to anyone else. Silverman is careful to note that winning the lottery is also unlikely, but it happens. To assure that a defendant receives a fair trial, the fallibility of fingerprint identification is something that fingerprint experts should be willing to acknowledge.
Admitting the possibility of error (even if the possibility seems remote) does not detract from an expert’s credibility. Rather, it enhances credibility. Nobody’s perfect. Juries understand that, and juries are more likely to trust experts who acknowledge their own potential fallibility. A recognition that experts are advocates for the truth, not for a particular party, always makes an expert a better witness.