They've been with you since before you were born, but how much do you really know about the lines and ridges on your fingers, palms, and feet?
1. THEY’RE THE RESULT OF A STRUGGLE.
Human skin has several layers, and each layer has sub-layers. A developing fetus is constantly straining and stretching these layers, which can snag on each other. Scientists believe fingerprints form when the bottom layer of the epidermis grows at a different rate than the rest of the skin, causing it to buckle and tug on the dermis. Your fingerprints are made up of several skin layers twisted together [PDF], kind of like a soft-serve swirl.
2. BEFORE FINGERPRINTS, THERE WERE BONE MEASUREMENTS.
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Alphonse Bertillon was a French policeman and researcher who capitalized on the fact that each person’s body proportions are different. He developed a way of using photographs to measure a person’s unique dimensions—a technique that’s still reflected in jailhouse mug shots. The Bertillon System, as it came to be known, was adopted by law enforcement agencies in Europe and North America and used for three decades.
3. SOME PEOPLE ARE BORN WITHOUT THEM.
Three genetic conditions can prevent fingerprints from forming: Naegeli-Franceschetti-Jadassohn syndrome (NFJS), Dermatopathia pigmentosa reticularis (DPR), and adermatoglyphia. NFJS and DPR cause a range of symptoms, most much worse than smooth fingers. Adermatoglyphia, on the other hand, has just one indicator: no fingerprints. It’s sometimes referred to as “immigration delay disease,” for the trouble it causes people trying to cross borders.
4. THEY KILLED THE BERTILLON SYSTEM.
In 1901, a man named William West began a life sentence in the Leavenworth, Kansas, penitentiary for murder. His Bertillon measurements were taken and dutifully cataloged. Two years later, Will West entered Leavenworth. When asked if he’d been there before, he said no, but the clerk took his measurements and photograph and found that they were an exact match for the man listed as William West who was currently in the prison. Befuddled, the clerk compared Will’s fingerprints with William's and found that, indeed, they were two completely different men. The story is still a matter of debate—some think the men might have been twins—but it soon became folklore among forensic scientists, illustrating not only the advantages of fingerprinting but the fatal flaws that would lead to the abandonment of the Bertillon system.
5. FINGERPRINT ANALYSIS IS FALLIBLE.
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When examining fingerprints, experts attempt to match as many points of comparison as possible, but there’s no minimum for a match, at least not in the United States. Other countries have set standards for what constitutes a positive identification, but not us. On top of that, there’s an inevitable element of human error. A 2011 study [PDF] found a false positive rate of 0.1 percent. That may not sound like much until you realize that 0.1 percent of the FBI’s annual fingerprint intake is 60,000 people, or 60,000 potential false positive IDs.
6. KOALAS HAVE THEM, TOO.
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So far, we’re aware of only a few non-human animals with unique fingerprints, such as gorillas, chimpanzees, and koalas. Given apes’ and koalas’ arboreal lifestyles, scientists suspect fingerprints evolved as a consequence of living in trees. The fingerprints of koalas are so similar to humans’ that even experts have trouble telling them apart. We haven’t heard of anyone blaming their misdeeds on a koala yet, but it’s probably just a matter of time.
7. FINGERPRINTS ARE INCREDIBLY DURABLE …
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Even in death, our fingerprints stick around, which makes them very helpful in identifying bodies. Or fingers, in the case of Hans Galassi. After losing a few fingers in an accident on the water, the wakeboarder figured they were gone for good. Then a human finger turned up in the belly of a trout and, sure enough, it was one of Galassi’s. “If a hand is found in water you will see that the epidermis starts to come away from the dermis like a glove,” fingerprint expert Allen Bayle told the BBC. “This sounds gruesome, but if a hand has been badly damaged, I cut the epidermis off and put my own hand inside that glove and try to fingerprint it like that.” (Once the severed finger had been identified, it was offered to Mr. Galassi, who declined to take it back.)
8. … BUT YOU CAN LOSE THEM …
Rough tactile work like bricklaying and chemotherapy drugs like capecitabine can erode and even erase fingerprints. “Just a good case of poison ivy would do it," forensics expert Edward Richards said in Scientific American. Don’t get too worried: "Left alone,” he said, “your skin replaces at a fairly good rate, so unless you've done permanent damage to the tissue, it will regenerate."
9. … ESPECIALLY IF YOU’RE DETERMINED.
By the 1930s, fingerprint analysis was standard practice in U.S. law enforcement, and criminals had begun intentionally trying to remove their fingerprints. As you might imagine, the results were grisly and mixed. Some tried to file off their prints, while others attempted to cut them out. Notorious gangster John Dillinger burnt his own prints off with acid, a hardcore decision that kind of worked. (His fingerprints were never used against him, but after his death the faint traces of his former ridges and whorls could still be seen.) Robber Robert Phillips talked a doctor into grafting skin from his chest onto his fingertips. Unfortunately for him, he neglected to remove the prints on his palms.
10. FINGERPRINT SENSORS MIGHT WORK FOR YOUR PETS, TOO.
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Apple created quite a buzz in 2013 when it introduced a fingerprint-coded screen lock with the iPhone 5s. Some of that buzz soon focused on cats, however, after a TechCrunch writer “commandeer[ed] a cat” and used its toe pad to create a new profile. “The cat’s paw worked,” he wrote, “and while it encountered more frequent failures than did a fingerprint, it was able to unlock the phone again repeatedly when positioned correctly on the sensor.”
11. MARK TWAIN ANTICIPATED THE VALUE OF FINGERPRINT EVIDENCE.
Two of the author’s books, Life on the Mississippi and Pudd’n Head Wilson, feature the use of fingerprints to nab criminals. Twain’s focus on fingerprinting was incredibly prescient; the books were published in 1883 and 1893, respectively, but U.S. officials wouldn’t implement fingerprinting practices here until the early 20th century.
12. WORLD WAR II SAW A BOOM IN FINGERPRINT COLLECTION.
Wartime vigilance meant that the FBI was collecting more prints than ever before, from soldiers, foreign agents, and military suppliers, as well as draft dodgers and potential spies. By 1943, the collection included more than 70 million prints. To manage the explosion of information, the agency moved to a big warehouse (nicknamed the “Fingerprint Factory”) and hired and trained thousands of women to sort prints 10 hours a day, six days a week.
13. THERE HAVE BEEN SEVERAL CASES OF MASS FINGERPRINTING.
In desperate times, British police have resorted to desperate measures. The shocking murder of a three-year-old girl in 1948 inspired officials to demand prints from more than 40,000 local men. Even with all those prints, they failed to find a match—until they tracked down the 200 men who had failed to produce prints. Among them, they found their culprit. Since then, despite protestations from Britain’s National Council for Civil Liberties, the police have conducted several mass print collections, several of which were successful.
That sort of thing doesn’t go over too well in the United States, but it has been done. The Fourth Amendment restricts the use of fingerprint collection to “reasonable” identification of persons of interest in criminal cases. Law enforcement officers could get around this if they chose, but it wouldn’t be a popular move.
14. THE FBI STORES EVERYBODY’S PRINTS TOGETHER.
If you’ve ever applied for a teaching job, the police force, or any government position, the FBI has your fingerprints—and they’re treating them like a criminal’s. In 2015, the agency announced that they were melding their criminal and civil fingerprint databases. They also decided to make all files searchable for potential culprits.
15. THE MICROBIOME IS THE NEW FINGERPRINT.
Like the whorls and loops of your fingerprints, the tiny ecosystems in and on your body are yours and yours alone. The collective DNA of the bacteria, fungi, and viruses that make up your microbiome is a huge repository of information about your health, environment, diet, and genetics—and it’s completely unique. Forensic scientists are currently exploring the possibility of using microbiomes for identification, and testimony based on microbial forensic techniques has already been admitted in some U.S. courts.