A pair of experiments presented at the recent annual meeting of the prestigious American Academy of Forensic Sciences demonstrated what could be serious limitations in crime scene DNA.
We tend to think of DNA as being transferred to objects only after significant handling and then remaining on those objects until the police come along with a DNA swab. Watching “CSI,” we’ve sometimes assumed that DNA at crime scenes can lead us to firm conclusions about whether a particular person was at the crime scene and when.
That may not be true.
The two experiments indicate that DNA could be transferred much more casually than we thought and could be much harder to draw conclusions about.
A 10-second handshake could transfer your DNA to an object someone else touches
In one of the experiments, DNA was transferred from one person, through another by handshaking, and to a knife. The first person never touched the knife directly, but after a 10-second handshake, their DNA was transferred to the second person and through them to the knife. This led to the first person depositing significant DNA about 7 percent of the time.
DNA of all sorts ends up on communal pitcher, individuals’ cups
In the other experiment, a group of people were set up in a restaurant-like atmosphere. The main participants had their own cups but shared a communal pitcher of water. Others were positioned throughout the room to simulate the restaurant conditions, and they could move around, talk and even leave, but they could not touch the pitchers or cups.
Whenever the main participants handled a cup or the pitcher, the object was carefully swabbed for DNA and those swabs were later tested for identification.
The researchers were apparently trying to see if they could tell who was the last person to touch the pitcher based on the DNA they left behind. They may also have been looking for whether any DNA from the pitcher was cross-transferred to any of the cups.
Surprisingly, DNA from the main participants ended up not only on the pitcher but on other participants’ cups, even though they had not touched one another’s cups. Even more surprising, DNA from the others in the room appeared on both the pitcher and the cups even though they had never touched any of the objects. This may have occurred when the others talked, coughed or sneezed, as these cause people to spew tiny droplets.
The researchers could not tell from the DNA swabs who handled the pitcher last or who handled it longest or most often. This may be because people shed DNA at different rates.
The idea that people can leave behind DNA on objects they have never touched is a novel one. “We’re all still trying to get a handle on how realistic this is,” admitted one researcher.
Yet if DNA can get left behind by people who merely pass through a room or shake hands with someone who goes on to touch a crime scene object, that could change much of what we know about DNA evidence. If police and prosecutors want to tie people to crime scenes through DNA left on objects, they will now have to overcome the possibility that any DNA they find could be that of a mere passerby. In some cases, this possibility could be enough for reasonable doubt.