Fingerprint Misconceptions

While writing The Hungry Bones, the fifth Alexa Glock forensics mystery, I read Brandon Garret’s Autopsy of a Crime Lab: Exposing the Flaws in Forensics. The book changed the way I (and Alexa Glock) view forensics. Misuse of the science has sent many innocent people to prison.

Brandon, a law professor at Duke University and director of the Wilson Center for Science, kindly answered my questions about fingerprints so that I can get it right in my books. (From The Hungry Bones onward, Alexa Glock will never utter that two fingerprints ‘match.’)

SARA: What are some misconceptions about fingerprints?

BRANDON: For decades, forensic analysts of many different types testified they were 100 percent certain, including fingerprint examiners. Most of us believed them. Yet, no one had carefully tested the basic assumptions that fingerprint experts have relied upon for decades. First, are each person’s fingerprints unique? In jury studies, I’ve found that about 95 percent of people believe fingerprints are unique. People think fingerprints are like snowflakes. Fingerprint examiners similarly assumed that all fingerprint patterns are completely different from each other, and not just that they are somewhat or mostly different from each other. Experts made the same strong assumption about bite marks, fibers, toolmarks, shoeprints, and a range of other types of forensics. We do not know if that strong assumption is true; it has never been tested.

Second, how often can one person’s fingerprint look like another person’s crime scene latent print? We do not know how often a smeared, partial latent fingerprint from a crime scene might look very much like someone else’s print. It may depend on what level of detail one has in a print. Errors can happen.

Third, how good are experts at making fingerprint comparisons? We need to know the error rates; after all, we are trusting experts to make decisions that can send people to prison or death row. It would shock jurors to hear of either a 1 in 18 or a 1 in 306 error rate. When a public defender in Joplin, Missouri, asked prospective jurors in a 2018 case about fingerprint evidence, they said things like, “I believe fingerprints are 100 percent accurate,” and “fingerprints are everything when it comes to a crime scene,” and “I mean, it’s an identifier . . . We’ve been taught all our lives that fingerprint is what identifies us, and that it is unique.”

SARA: How does bias interfere with a fingerprint examiner’s findings?

BRANDON: There is greater understanding today that policing can be biased, including racially biased, in ways that disproportionately burden minorities with criminal legal involvement. Forensic science plays an under-appreciated role in that system. In forensic science, testing is rarely blind.

Traditionally, the forensic examiner talks to the police, and hears all sorts of biasing information about the suspect, like the person’s race and criminal record. Forensic experts are also biased by the side that they work for. Experts who see themselves as part of the police and prosecution team predictably shade their results to help the prosecution. Experts are also biased by the side that hires them. If forensic examiners at crime labs feel that they are retained only by police and prosecutors, they will tend to view their role in a different way than an expert hired by the defense. This has troubling consequences in criminal cases, where often the defense does not receive any funds for an expert.