Rules of Thumb: Writing About Fingerprints

We all leave fingerprints behind every day. Anyone who wears glasses or owns a mirror has likely needed to clean them off. No matter how often we wash our hands, a natural buildup of grease on our skin means that we can’t stop leaving fingerprints. It may be annoying to clean, but they can get criminals in a world of trouble.

Let’s talk about how law enforcement (and writers) use fingerprints to finger the culprits.

The History of Fingerprinting
People figured out early on that fingerprints are a good way to identify someone. Ancient civilizations used fingerprints as seals or signatures on important documents and contracts long before paper was invented. However, the idea of using fingerprints in criminal investigations didn’t happen until much later.

Not until 1840 did Dr. Robert Blake Overton suggest to Scotland Yard that checking for fingerprints might help them solve the highly-publicized murder of Lord William Russell. Even then, most police forces remained skeptical. Over 50 years later, Argentina became the first country to create a method of recording individuals’ fingerprints and keeping them on file. Other countries didn’t follow until the early 1900s, but they have embraced the practice wholeheartedly.

Lifting Prints
Everyone has seen detectives break out the powder and brush to dust for fingerprints. By spreading dust on the surface, the dust sticks to the grease, making a fingerprint easier to see and record. But powder isn’t the only option; it’s just the easiest.

There are two main challenges in lifting prints: how complete the print is and the surface holding the print. If you’ve ever touched something accidentally, you know people don’t tend to press a finger down completely, so technicians often work with partial or latent fingerprints. Additionally, certain textures simply don’t hold fingerprints. A surface must be a bit porous to hold residue; otherwise, the print could smudge.

Matching Prints
Fingerprint identification isn’t an exact science. In theory, comparing the ridges, whorls, and loops between two prints should allow someone to determine whether or not they come from the same person. But, it’s rarely that simple.

What if you lifted a left pinkie print at one scene and a right thumb at another? Luckily, new research has found that you can probably still identify whether those two fingers belong to the same person.

Typically, the print is first scanned and run through a computer. The computer can compare fingerprints much faster than a person and assign each match a likelihood (in percentage). Then, an expert will look at the multiple matches to identify the most likely match (if there is one).

However, dermatoglyphics has been frequently criticized as being overly subjective, and different departments have different standards to meet to declare a match. The average fingerprint has 150 ridge characteristics, yet there is no universal standard of points. That means one expert could declare a match with 10 characteristics in common and another with six.

Are Fingerprints Unique?
The answer is technically no. While it is possible for two people to share a single fingerprint, the likelihood of it happening is estimated to be less than one in 64 billion. So, in theory, someone might wind up with the same left middle fingerprint as someone else; it’s just extremely unlikely (even identical twins have different fingerprints). It's unlikely enough that investigators and governments have been relying on fingerprints for hundreds of years.