In exonerating the wrongfully convicted, DNA is often a conclusive way of proving a person’s innocence. DNA testing on biological evidence left behind by a rapist or murderer can show that the actual culprit’s DNA does not match the DNA of the person who was convicted of committing the crime. When that happens, we know that an error has been made and that the person who was convicted is innocent, which helps in the fight to free the wrongfully convicted. But, surprisingly, the flipside is not true – instead, there are real problems with trying to use DNA evidence to prove guilt. We have to be careful because sometimes when DNA evidence is used to prove that a person committed a crime, mistakes are made and that seemingly reliable evidence can lead to errors. Why the double standard?
In the wrongful convictions context, exonerating DNA material is typically unquestionably from the perpetrator: blood from a struggle, semen or saliva from a rape, skin cells under the victim’s nails from a struggle. When that kind of DNA evidence does not match the person convicted, we can assume with certainty that the conviction was wrongful. But DNA testing has now advanced to include something called “Touch DNA,” and that methodology leaves plenty of room for errors when it is used in the opposite context, to try to prove a person’s guilt.
“Touch DNA,” also called “Contact Trace DNA,” is DNA that is recovered from skin cells left behind when a person touches items such as clothing or a weapon. People shed about 400,000 skin cells per day, and these cells can be lifted from weapons or objects found at the crime scene and then tested for a Touch DNA sample. But the problem with using Touch DNA to prove guilt is that a person’s DNA does not stay in one place – skin cells move. For example, I can borrow a scarf that my friend wore, and when I bring it somewhere, I can transfer her skin cells to someplace that she has never been or to an object that she has never touched.
As an example, based on DNA evidence a homeless man named Lukis Anderson was charged with capital murder for the death of a Silicon Valley multimillionaire. But there is no way that Anderson could have committed the murder – at the time of the murder he was hospitalized, nearly comatose, and under constant medical supervision. It turned out, however, that the paramedics who treated Anderson were the same ones who responded to the murder scene. The paramedics accidentally “planted” Mr. Anderson’s DNA at the scene hours after assisting him. That case has become the leading example of the risks of the criminal justice system relying on DNA evidence as infallible proof of someone committing a crime.
In another alarming example, a study published in the Journal of Forensic Sciences recently concluded that a person who uses a steak knife after shaking hands with another person transferred that person’s DNA onto the handle in 85% of the samples examined. Thus, a person’s DNA on a murder weapon does not necessarily mean that he or she was the one who handled it. Another study found that fingerprint brushes used at crime scenes to find latent fingerprints could actually be picking up and then dropping Touch DNA from one crime scene to the next.
These are scary realities. People often think of DNA as being infallible – and it truly can conclusively prove the innocence of the wrongfully convicted. But we have to be very careful about using the same type of evidence to prove a person’s guilt.