We've all seen police dramas where triumphant suspects are appalled to learn that their fingerprint is on, say, the underside of the door handle of the victim's car. But what qualifies as a 'matching' print? And is the system always perfect, or does it make mistakes? Take a look at how incriminating your fingertips actually are.

Top image: Shutterstock.com

These days, fingerprinting people seems almost quaint. With a DNA database, fingerprints seem like something Dick Tracy would rely on . But there's more power in a fingerprint than many people think — and there may be more power in them than DNA.


For example, James and John Parr were both arrested for stealing 10,000 Pounds worth of watches, after they left some blood on a pane of glass. They were identical twins. The DNA matched each of them and each twin said they were home at the time of the burglary. If either one of them had left a fingerprint on the glass, the police would have been able to arrest the guilty twin. Although twins have similar fingerprints, it's easy to distinguish between them. With only DNA, the cops were helpless. Since both people can't be arrested for a crime only one of them did, they were let go.

But at the same time, a person's fingerprints get stretched, scarred, and bent out of shape. How much of a print is a conclusive print? And how does the process work?

What Fingerprints Are and Why Identical Twins Don't Have the Same Ones?

There's no doubt that genetics does play a role in fingerprint production. The tiny ridges and whorls on a finger follow patterns set out by DNA. Why your DNA should bother with creating these at all is sort of a mystery. Some people say that fingerprints increase your ability to grip surfaces. Others contend that the way the tiny ridges interact with the environment help people feel texture.


The actual things that creates these ridges are the dermal papillae. These are little 'pegs,' that anchor the dead outer skin to the sensitive, living inner skin. The pegs are almost always there, but they can be moved around quite easily when we're young. Studies have found that touching amniotic fluid from week six to week thirteen in a pregnancy can change the fetus' eventual fingerprint pattern. Add to that slight differences in nutrition, motion, and growth spurts, and two kids with identical DNA can have fingerprints that are easily distinguishable. Although there is disagreement on the exact odds of overlapping fingerprints, some put the chance that one person's prints will match another's at one in sixty-four billion.

Why do we leave them on nearly every surface we touch? Well, because we're disgusting greasy balls of sweat, of course. Each of these ridges has pores underneath. Each pore has sweat glands. Each sweat gland churns out enough ooze to let the fingertip swim in a swamp of it one surfaces. The ridges push away some of the gloop, and leave impressions that can be measured.


How Much of a Fingerprint Can Convict You?

It's interesting to think that you are carrying around your conviction on your fingertips. But what exactly do the police need? Well, they already have more than most people think of. Robert Phillips, a gangster, famously had a plastic surgeon remove both layers of skin from his finger tips. Great idea — but the only problem is, finger prints extend past the first joint. The ridge pattern goes down the finger, and many law enforcement agencies take the print of the entire finger. Phillips was convicted when they matched the ridges of his second joint to prints they found at the scene of crimes.


Many aspiring felons will be wondering how little of a print can be left at a scene for them to walk out a court room. It really depends on how many details that a person, or a computer, can squeeze out of a print. While there are only a few overall patterns on prints, each print has shapes that appear at certain places. Three ridges coming together is a delta. The center of a circle or peak of an arch is a core. A ridge branching out into two ridges is a bifurcation. A single ridge tiny enough is a dot. Computers spot these landmarks and mark their location, checking them against other fingerprints. About twelve points is considered enough to declare the prints a match, but a careful examination of a good print can yield up to fifty matching points.

How Uncertain are Fingerprints?

Say you have a print that's got a one sixty-four billion chance of being duplicated and it's been matched twelve ways from Sunday — what possible question could there be? And yet, there are mistakes. In 1998 a Massachusetts man was convicted of a murder he didn't commit based on prints that were incorrectly matched. He spent six years in prison before being exonerated. In 2004 a lawyer in Oregon was jailed for two weeks when his prints were incorrectly matched with those found at a site of a train bombing in Madrid. How does this happen?


For one thing, it's somewhat rare for a completely un-smudged print to be found. The blurring of a smudge leaves a lot of guesswork, which gets tricky when labs confirm from the top down. When asked to confirm a boss's work, underlings are unlikely to disagree. There's also the problem of certification. Many labs don't require any. There have been several certification exams sent around. One, sent out regularly by a company called Collaborative Testing Services, sees a failure rate, depending on the lab, of three to twenty percent. And those are complete prints. The average print recovered from a crime scene is roughly twenty percent of a finger.

And even if the technicians match up the prints, there's no official standard for what is declared a match. Some countries require a meager seven points, some twelve or fourteen, and some thirty. A jury being told that fingerprints 'match' don't get the difference between seven points and thirty points. They just hear that the prints are the same.


In the end, the reason why fingerprints seem old-fashioned when compared to DNA, especially now, is because they are. Sure, they may be more accurate, but they came of age in an era well before our time, and were grandfathered in to the modern court system. DNA had to earn its merit in the current court system, which means stricter and more rigid criteria. Fingerprints, what with their ability to sidestep the "twins" problem, are more unique than DNA, but DNA, as a product of modern standards, is more accurate. To bring fingerprinting into the modern era, it needs to become more accurate, too.

Second Image: Frettie

Elements of a Fingerprint Image: DoD

Fingerprints of Rosa Parks: National Archives

Via Slashdot, Discovery, BU, How Stuff Works, TIFO, USA Today, Viewzone, and CBS News.