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DNA can be transferred to objects via handshakes, at random

Could ordinary contact with other people spread your DNA to objects you've never touched?

Yes, according to an important presentation at a recent meeting of the American Academy of Forensic Scientists, an internationally prestigious event held each year. About 7 percent of the time, DNA was transferred to a knife by a person's handshaking partner. This occurred after only a 10-second handshake.

Previous studies revealed that "innocent transfer of DNA" could occur, but they were criticized by some as unrealistic because they involved longer contact, such as a two-minute handshake. This study addressed that criticism by using a much shorter handshake for only 10 seconds. 

Researchers find DNA of all sorts of people in communal experiment

A separate study by a forensic anthropologist at the University of Indianapolis found that the last person to touch an object was frequently not the person who left the most DNA behind.

In that experiment, students were given cups and a communal pitcher to drink from. Other students were present to simulate a restaurant-like atmosphere. As each student handled the pitcher and cups, researchers swabbed those objects for DNA. Even though the students only handled their own cups, DNA from other students was found on those cups and on the pitcher.

Even more startling, DNA from students who never touched the cups or the pitcher was found on the pitcher and cups. The researchers surmised that the DNA had been transferred via airborne droplets released when the students spoke, coughed or sneezed.

Ultimately, the researchers were unable to determine, based on the quantity and location of DNA on the pitcher and cups, who handled the pitcher last or how long any given person had touched a particular object.

At least in a social setting, the DNA transferred easily and unpredictably from person to person and to the objects.

These findings challenge the assumption that any DNA left on objects at a crime scene must belong to the perpetrator or someone else who was present. Now, we know that a person one step removed -- someone who shook the hand of someone present at the crime scene -- could be the source of the DNA.

DNA left behind on objects does break down over time, or investigators might be inundated by unrelated DNA at every crime scene. However, it is unclear just how quickly it breaks down or how likely it is that randomly deposited DNA will appear at a given crime scene.

A forensic geneticist from the John Jay College of Criminal Justice says that it's probably rare to find a person's DNA on an object they've never handled, although she did not say why she concluded that.

"We can't discount [the idea]," she said, "but we shouldn't use it to throw the evidence out in every single case."

Of course, no one advocates abandoning DNA in every single case. Yet if DNA can indeed be deposited on items you've never touched, that might be enough for reasonable doubt in some cases.

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