The Trouble with Children's Prints

In 2nd millennia BCE, people in Babylon who wished to enter legal contracts together would leave plastic fingerprints (physical impressions of their fingertips) in clay tablets. 

In the first millennium CE, people in China used patent thumbprints (where an ink was purposefully transferred onto a document by a thumb) to sign documents such as loans and divorce papers.

In the 1300s, a Persian physician wrote in a text that fingerprints were probably unique to everyone.

In the early 1800s, scientists began acknowledging the uniqueness of fingerprints. Francis Galton calculated that the odds of two people having the same prints are 1 in 64 billion, while Paul-Jean Coulier discovered in 1863 that iodine fumes can make latent prints (the invisible prints of skin-oil known as sebum, which are left behind on objects that people have touched) on paper visible to the naked eye.

Yet it wasn't until the 1800s that modern police started acknowledging the usefulness of fingerprints for identifying criminals. Indian and Argentinian police began collecting exemplar prints (the ink fingerprints purposefully taken for identification purposes) of criminals in the late 1800s. 

Finally, in France in 1902 Alphonse Bertillon (inventor of the mug shot) first used fingerprints to catch and convict a criminal of murder, after he was able to compare the prints found at the crime scene with the exemplar prints police had taken from the criminal weeks earlier after being arrested for another crime.

Plastic fingerprints in clay, patent fingerprint of grease, and latent fingerprints on a cell phone screen

By the early 1900s, fingerprinting became the norm for police performing routine crime scene investigations.

But in 1995, a strange discovery was made. Children's latent prints seem to disappear over time. In fact, while adult latent prints remain unchanged for months or even years, children's prints begin to disappear within hours of touching an object. This, of course, poses a number of problems, the first being that proving that a missing child had been in a particular location would be all the more difficult, and could hamper police investigations of missing children.

Scientists began testing to determine why this occurs, in the hopes that if it was understood they would be able to discover a way to prevent valuable clues from being lost. 

A child's patent handprint in paint

Studies have shown that while all latent prints consisted of sebum, the components of the sebum differs between children and adults. Children's latent prints have lower concentrations of lipids, waxes, and glycerides, and higher concentrations of cholesterol and cholesteryl acids than adults' prints. Because cholesterol and fatty acids are unstable, they begin to break down more rapidly making the latent fingerprints disappear. It was therefore hypothesized that targeting the carboxylate acid salts might result in the recovery of children's prints.