DNA’s Itinerant Nature

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.

21 thoughts on “DNA’s Itinerant Nature

  1. Ken Mackenzie

    Farrah Jama’s DNA was on a rape kit swab, taken from a woman he had never met. His conviction was quashed in 2009.

    1. SHG Post author

      What an amazingly important bit of information. Perhaps we should go through every instance, however many billions of them, when DNA traveled, because it adds so much for those readers incapable of grasping the implications of this post without a concrete example and a name to go with it. Did you notice how I studiously omitted the standard-issue anecdote used as a rhetorical device in the Marhsall Project post designed to draw in the emotionally-charged but intellectually-challenged reader? Why, oh why, would I deliberately leave out the sad story?

    2. Chris Halkides

      Frank Vincent was a retired justice who led an inquiry into the Farah Jama case.

      In other words, the DNA evidence was, like Ozymandias’ broken statue in the poem by Shelley, found isolated in a vast desert. And like the inscription on the statue’s pedestal, everything around it belied the truth of its assertion. The statue, of course, would be seen by any reasonably perceptive observer, and viewed in its surroundings, as a shattered monument to an arrogance that now mocked itself. By contrast, The DNA evidence appears to have been viewed as possessing an almost mystical infallibility that enabled its surroundings to be disregarded. The outcome was, in the circumstances, patently absurd.

      The Vincent report makes other good points about DNA contamination IMO.

  2. Skink

    I’m not at all sold on the theory that resolution of this is for science. Wide and easy transfer of DNA has been known for decades. Every DNA scientist is keenly aware of that science and the various modes it takes. Frankly, the science is simple. Traditionally, it results in mixtures. But the testing has become so sensitive that stray DNA can be detected. Again, DNA scientists know this to be true. That doesn’t seem to be a science problem, and the article doesn’t suggest what type of study should be undertaken.

    This as a lawyer problem. If DNA is evidence, treat it as any other evidence by investigating how it got there–just the same as probing the protocols in the lab and handling by techs at the scene.

    How I can shed 5,000 square yards of skin cells in two minutes also causes me thought seizures. I must be bigger than I thought.

    1. SHG Post author

      Is the question whether science should do something to solve it or whether criminal defense lawyers should know about it in order to better understand the problem they’re facing. Remember, this is a criminal law blog, even though we are graced by your civil lawyer presence and 5,000 square yards of skin cells. Ew.

      1. Skink

        Criminal defense lawyers should know it and should have known it because it isn’t new. To the extent the referenced article suggests additional scientific study on how DNA moves is the answer, I say nope because it’s known. There isn’t even a mention of what that study would seek.

        I know it’s a criminal defense blog, and I wouldn’t dare inject my civil semen into the issue, but DNA ain’t just for CDLs. It’s everywhere!

        1. SHG Post author

          Yes, they should. No, many don’t, because they never deal with DNA in their practice. But now that you’ve told them how stupid and incompetent they are, it kinda makes me seem foolish for writing this post. I’m now going to ball up in a corner and try to block the thought of your civil semen from my mind.

          1. Skink

            Success comes in many forms. I’m glad I had a meager measure before lunch. Besides, you know you goaded me to do it.

  3. Joe

    If you really want to open up a can of worms, let’s talk about what happens when DNA proves the accused was innocent. DA’s have a litany of excuses to explain why they are still guilty.

    1. SHG Post author

      Or we can focus on the limited issue of the post and not go shooting off blindly in the billion orthogonal penumbras that are routinely enjoyed by the smartest people on reddit. So let’s keep that can of worms closed, k?

      1. Skink

        Does the celebration call for specific hats and drinks, or can I just leave additional samples around town?

  4. Billy Bob

    My first thought here is that CDLs shed 100 M pieces of DNA on a good day, and 200 M on a bad one. You CDLs sweat like warthogs. I’ve seen you guys in action, trying to make a living in the well of unpredictable courtrooms across Amerika, the land of the freeloaders and Home of the Brave New World. Ill-fitting suits and uncomfortable shoes. We know your type.

    Forgive me if I repeat myself: Old lawyers never die, they just loose their appeal. You ever notice how easy it is to lose an appeal? I mean, it’s “baked-in-the-cake.” Winning an appeal is like,… well, hitting Lotto. Let’s face it. The appellate courts hate us. They just want us to disappear. They will say anything to make us go away, and they do. Trust it.

    We have our own anecdotal and personal observations of the utmost importance. However, based on the above drubbing of Skink (and other poor slobs over the years), we’re holding back; pending the publication of our own best-selling book–which has yet to be written and/or edited by people smarter than me. Let me say this about DNA: It’s not what it’s cracked up to be, nor was it ever. However, life goes on, and prosecutors are loathe to forgo the low-hanging fruit of easy conviction, no matter how science-y or not the so-called evidence might be. Over and out.

    Have a good day, Mate. What ever happened to Court TV? We were addicted there for some time, way back when TV was popular. It was better than Judge Judy, much better. Trust it. Real court, when the cameras are not rolling, is always a nitemare–even when you win. Trust it. Heads they win; tails, you lose. Somehow they manage to make an end run around Constituitional rights, laws, statutes, case history, precedent and/or rules of procedure. I don’t get it,… but then again IANAL. So how could I?!?

    But I did stay at a Motel 6 last nite, and my new handle is F. Lee Billy, for those who have not noticed. Yea, F. Lee is/was the P. T. Barnum of the U.S. justice system, if that’s what you wanna call it. What a farce you people are!

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