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.

Help From Experts

I love doing research for the Alexa Glock forensics mysteries. I am not an odontologist like Alexa, or a forensic investigator (except in my fantasies!), yet my mysteries center around them. Here are two ways I achieve credibility: I do careful research and I have experts in the field read over what I write. These experts are my rock stars!

While writing The Bone Track, I began working with a forensic science researcher who specializes in fingerprint comparisons. Here are two ways she helped me:

In a draft of The Bone Track, Alexa thought the smooth cork handle (of a hiking pole) would be rife with fingerprints. After reading this, my expert said: “Cork would be a horrible background (we call it “substrate”) because it is so patterned and that pattern will pick up the fingerprint powder and be pulled up by the tape.” The lifted prints would have a distracting pattern running through them that would make them difficult to see.

Also in an early draft, I had Alexa dipping a dead person’s fingers in hot water to break rigor so she could take fingerprints. My expert said, “I love that you brought in hand-boiling method! This is a fairly new method that is still gaining widespread knowledge and acceptance and it is so cool. That said, you’re not using it quite right. Hand-boiling is not used to break rigor – it’s used to plump the ridges back up and make them recordable when the body has been waterlogged.” She gave me suggestions for breaking rigor, which included using brute force. She said, “ I’ve had to physically jump up and down trying to break rigor on someone before.” (See how Alexa does it in The Bone Track!)

I am working on the 4th Alexa Glock novel. A forensic pathologist in New Zealand is reading my scenes. She is amazing, too! She has helped me understand New Zealand’s death investigation system.

She pointed out that the timeline in Book 4 and the extent of decomposition in the body don’t align: “You can have the rate of decomposition sped up by a higher temperature in the bunker, which would be believable if the ventilation system has been plugged up. You could also have the body with the head downward, which would cause more blood to pool in the face and speed up decomposition in the face.”

Done and dusted! (YOU now have a sneak preview of The Bone Riddle.)