How DNA Evidence Went From Airtight to Error-Prone

Blind faith in any technology can be dangerous -- especially when it comes to areas of forensic science such as DNA fingerprinting. For example, if police have “DNA evidence” against a suspect, most juries will assume that’s proof of guilt. But while the technology for analyzing DNA has become vastly more sensitive since it was first introduced in courts in the 1990s, crime labs are working with ever more minute traces -- sometimes just a few molecules -- and drawing inconsistent or erroneous conclusions from them. In fact, there’s good reason to believe DNA evidence has sent people to prison for crimes they didn’t commit.

That’s the conclusion of a recent report commissioned by the President’s Council of Advisors on Science and Technology (PCAST), which called into question the increasingly common practice of analyzing mixtures of DNA from several individuals. Police can now collect DNA not only from blood stains and other bodily fluids, but from traces of cells left behind when people touch door handles, guns or other objects associated with crime scenes. These so-called “touch DNA” samples often include a mixture of genetic material from multiple people who handled those items.