Does DNA at a crime scene prove guilt?

You are a member of the jury for a high-stakes murder case. The defendant pleads not guilty, and you have not been convinced either way. But then, the prosecutor tells you that the defendant’s DNA was found on the murder weapon! Surely a guilty vote is obvious.

But think again. A report published this year in the Journal of Forensic Sciences suggests that DNA evidence might not be as clear-cut as one might assume. In the study, scientists led by Cynthai Cale and Madison Earll from the University of Indianapolis show that secondary DNA transfer—the indirect transfer of someone’s DNA to an object or person via an intermediate— is a real risk with the high-sensitivity technology available today that can detect DNA from only a few cells.

In their experiments, Cale and Earll had volunteers shake hands for two minutes before handling a knife. In almost all cases, DNA swabbing detected DNA from the person who did not directly touch the knife, and in one-fifth of samples that person was actually identified as either the only or the major contributor of DNA.

These findings have significant implications in forensic science, as not only can secondary DNA falsely place someone at the crime scene, but the results could also weaken evidence that could identify potential suspects. A 2013 case in California highlights the risk: DNA that was likely transferred to a murder victim from a responding paramedic led to the false accusation of a man who had been earlier treated by the same paramedic.

Cale and Earll’s research shows that DNA evidence by itself cannot be taken as a conclusive sign of guilt, and must be carefully interpreted like all evidence.