Your DNA can show up at crime scenes you were never at, study finds

Forensic DNA evidence is an invaluable tool for determining guilt and innocence in criminal investigations.

Technology has reached a point where sub-microscopic samples – so-called “touch DNA” – can determine, well within a reasonable doubt, who handled what at the scene of a crime.

But newer, even sharper technology may be about to call that into doubt.

It is now becoming possible to detect other samples – “transfer DNA” – from people who were never at the crime scene.

“Touch DNA usually refers to DNA that’s deposited by the skin onto an object if you touch it,” says Dr. Krista Latham, associate professor of biology and anthropology and the University of Indianapolis.

“However, our study is suggesting that term might be misleading, because what we found is that DNA can be transferred to an object that a person did not have direct contact with.”

In other words, your DNA can actually be found on objects you’ve never seen – and never, ever touched.

A police forensic expert gathers evidence from a crime scene in Lahore, Pakistan October 1, 2015. (Reuters)
A police forensic expert gathers evidence from a crime scene in Lahore, Pakistan October 1, 2015. (Reuters)

In a paper published in the Journal of Forensic Sciences, Latham and her team have fired a warning shot across the bow of DNA infallibility.

“We’re not trying to negate the value of DNA analysis at all, and not trying to say it’s not a valid investigative tool, because it absolutely is,” Latham told Yahoo Canada.

“What we’re trying to do is raise awareness that careful interpretations need to be made of the DNA profile.”

In the study, participants were asked to shake hands for two minutes. Frequently, this was long enough that, when one person picked up an object, the other person’s DNA was transferred to it as well.

Now, nobody ever shakes hands for two whole minutes, but we do hold hands that long. Or we get a massage, or physiotherapy – or we get intimate with someone. There are ways that this can happen, and Latham feels it certainly has implications going forward.

“We’re not suggesting that old cases need to be re-examined,” she stresses, explaining that technologies used in the past were not sensitive enough to detect this transfer DNA.

“One thing that I want to emphasize is that we are using the most sensitive technologies that are available. The ability to detect these very small amounts of secondary transfer DNA is going to be associated with these technologies.”

In other words, this is a potential problem going forward, not a game-changing one looking back.

But it does raise the question: has DNA evidence been relied on too heavily as a way of solving crimes?

“I think it’s going to continue to be very useful,” Latham says.

“The generation of a DNA profile is very sound science. And when you look at the history of forensic DNA analysis, it’s one of the most rigourously tested and validated of all the forensic science tools. I don’t think our reliance on DNA will change at all. What we’re just suggesting here is that we need to be careful in our interpretations.”

Meaning that, in what has been a black-and-white school of criminal evidence, a gray area is beginning to emerge.

“I think you’re going to see this same cautionary tale in many of the investigative tools that are being used,” Latham predicts.

“If you look at the tools to detect trace analysis, technologies everywhere are getting more and more sensitive. And the more sensitive these technologies are, the more likely they are to pick up background noise. And that’s all we’re suggesting here.

“As the science changes, so do our interpretations.”