Over the summer, the Texas Forensic Science Commission, which reviews the current standards for physical evidence in Texas criminal courts, discovered that state crime labs were using outdated protocols for analyzing DNA evidence. In particular, the problems occurred in “mixed DNA” test results, where the labs were exaggerating the likelihood that a particular sample matched a defendant’s DNA.
There are two types of DNA profiles that can come from any given DNA sample: “mixed DNA” (also called “mixtures”) and single-source DNA. Mixtures are the most common type of DNA profiles. A mixture occurs when a DNA sample contains DNA from more than one person. For example, if two or more people touched the handle of a firearm, a DNA sample from this handle would likely be a mixture of multiple people’s DNA. In contrast, a “single source” DNA profile exists when only one person contributed the DNA to a sample. In the case of single-source DNA profiles, matching a given person to the sample is similar to matching a person to a fingerprint left at a crime scene. However, because there are numerous types of DNA profiles that could combine to create a mixture, the probability that a random person besides the defendant could have contributed to this mixture is higher than in the case of single-source DNA profiles.
The Texas Forensic Science Commission recently revealed that state labs have been reporting inaccurately high probabilities of matches between defendants and mixed DNA samples since 1999.
An example of a recent Galveston murder case illustrates the problem. The crime lab reported that the defendant contributed DNA to a mixture from the crime scene, with a certainty of more than a million to one. However, it turns out that the lab was using outdated protocols, and the certainty is actually just forty to one. This means that if you selected 1,000 random people, 25 of them would match the DNA found at the crime scene. That’s hardly enough evidence to convict someone beyond a reasonable doubt.
Everyone knows that eyewitness testimony can be fallible, so prosecutors have placed great weight on scientific DNA evidence when prosecuting cases since the advent of DNA testing in the mid-1990s. Due to jurors learning about the infallibility of DNA science from television crime shows (the CSI effect), jurors often convict defendants on the basis of DNA evidence alone. Now that we know that scientists were inaccurately reporting the probability that crime scene DNA matched particular defendants, many convictions over the last fifteen years may have been based on false evidence, and innocent people could be behind bars.
Police labs and prosecutors’ offices have downplayed the severity of the problem. For example, according to a memorandum from Art Acevedo, the chief of the Austin Police Department, the inaccurate reporting is “unlikely to materially affect any assessment of evidential value.” Court orders will be required in order for the labs to re-test samples to determine accurate statistical probabilities of a match between crime scene samples and defendants’ DNA.
I’m concerned that prosecutors and police forces are sweeping this issue under the rug. Rather than re-assure the public with rhetoric, they should create task forces to revisit every case where DNA played a role in obtaining a conviction, and the labs should recalculate statistical probabilities in all of those cases. After all, it was the State that put false evidence before juries, so the burden should be on the State to correct the problem and regain the public’s confidence that innocent people aren’t behind bars.
If you or a loved one suffered a criminal conviction as a result of mixed DNA test results, you should contact Chris Perri Law to determine whether your case could have been affected by inaccurate scientific testimony. Check out the articles below for more information on this unique and unsettling issue: