So much of our beliefs about “who done it” is conclusively wrapped up in the gold standard of identification, DNA. It’s not just all scienc-y, but good, solid repeatable science. Sure, there are tons of quibbles about the statistical likelihood of DNA being from a different person, contamination, magic-box testing and whatever other screw-ups humans bring to the lab, but when we’ve got it, we can take comfort in knowing that science says we got the right dude. Or better yet, they got the wrong dude, an innocent person, so cut him loose.
But there remains another nasty problem with DNA, that it doesn’t like to stay in one place.
Back in the 1980s, when DNA forensic analysis was still in its infancy, crime labs needed a speck of bodily fluid—usually blood, semen, or spit—to generate a genetic profile.
That changed in 1997, when Australian forensic scientist Roland van Oorschot stunned the criminal justice world with a nine-paragraph paper titled “DNA Fingerprints from Fingerprints.” It revealed that DNA could be detected not just from bodily fluids but from traces left by a touch. Investigators across the globe began scouring crime scenes for anything—a doorknob, a countertop, a knife handle—that a perpetrator may have tainted with incriminating “touch” DNA.
And this opened a Pandora’s Box of new, very different problems, turning the gold standard upside down. It wasn’t that DNA turned out to be a sham, or that van Oorschot was wrong. It didn’t, and he wasn’t. It was the nature of DNA.
But van Oorschot’s paper also contained a vital observation: Some people’s DNA appeared on things that they had never touched.
In the years since, van Oorschot’s lab has been one of the few to investigate this phenomenon, dubbed “secondary transfer.” What they have learned is that, once it’s out in the world, DNA doesn’t always stay put.
We touch things. All of us. And the DNA on our fingers decides to hang around wherever we touch, as did the DNA of those who touched before us. Except the old DNA, bored with the scenery, decides to grab a ride on our fingers, clothes, and go see the world.
To find out the prevalence of DNA in the world, a group of Dutch researchers tested 105 public items—escalator rails, public toilet door handles, shopping basket handles, coins. Ninety-one percent bore human DNA, sometimes from half a dozen people. Even items intimate to us—the armpits of our shirts, say—can bear other people’s DNA, they found.
This isn’t a matter of our being careless with our DNA. Humans shed it. All the time. Huge quantities of it.
In a sense, this isn’t surprising: We leave a trail of ourselves everywhere we go. An average person may shed upward of 50 million skin cells a day. Attorney Erin Murphy, author of “Inside the Cell,” a book about forensic DNA, has calculated that in two minutes the average person sheds enough skin cells to cover a football field. We also spew saliva, which is packed with DNA. If we stand still and talk for 30 seconds, our DNA may be found more than a yard away. With a forceful sneeze, it might land on a nearby wall.
As you work through the possibilities, it becomes obvious that DNA can move from place to place, person to person, creating a near-chaos theory of connections that ultimately end up on a swab in a crime lab. It’s not that the swab is necessarily wrong, or that any of the plethora of other potential errors producing false results occurred. It’s just the itinerant nature of DNA.
The itinerant nature of DNA has serious implications for forensic investigations. After all, if traces of our DNA can make their way to a crime scene we never visited, aren’t we all possible suspects?
Our adoration of science, both for its putative conclusiveness as well as its alleviation of the dreaded responsibility of caging another human being, makes this a very significant problem. If the DNA says he did it, well then, he did it. Except nobody asked the DNA how it got there, but we’re going to believe it nonetheless.
This is partly because most forensic scientists believe DNA to be the least of their field’s problems. They’re not wrong: DNA is the most accurate forensic science we have. It has exonerated scores of people convicted based on more flawed disciplines like hair or bite-mark analysis. And there have been few publicized cases of DNA mistakenly implicating someone in a crime.
But, like most human enterprises, DNA analysis is not perfect. And without study, the scope and impact of that imperfection is difficult to assess, says Peter Gill, a British forensic researcher. He has little doubt that his field, so often credited with solving crimes, is also responsible for wrongful convictions.
The use of DNA to preclude identification remains one of the great, if highly limited, miracles of forensic science. The eyewitness may conclusive identify the perp from the stand, “I will never forget that face,” the witness proclaims in a clear and stentorian voice. Then the lab tech takes the stand and says, “the defendant was excluded as a possible person who committed the crime,” and everybody goes home.
But when DNA at the crime scene identifies a person, they’re going to get arrested and prosecuted. They can prove they were elsewhere? A scam! A sham! After all, why would a prosecutor believe ten witnesses that the perp was in a different country when the crime occurred. After all, the DNA said so.
“The problem is we’re not looking for these things,” Gill says. “For every miscarriage of justice that is detected, there must be a dozen that are never discovered.”
Maybe even more than a dozen. It might be a mistake to call our appreciation of DNA a fetish, as it has fundamentally changed the relative forensics of criminal prosecution, likely for the better. That said, there is no doubt that DNA likes to move around a lot, and refusal to know of, appreciate or accept the premise that just because some DNA was found does not conclusively explain how it got there is something that should not be ignored.