DPI’s “What to Know” series examines capital punishment from multiple angles, one topic at a time. Each installment provides essential facts and data on specific aspects of the death penalty. This installment looks at junk science: how flawed or overstated forensic evidence has contributed to wrongful capital convictions, and what happens when the understanding of evidence underlying a conviction changes.
Why this Matters:
Forensic evidence is often presented to juries as objective scientific proof, but advances in DNA testing and scientific research have exposed weaknesses in methods once treated as reliable. Subsequently, junk science in capital cases extends far beyond a singular forensic discipline. Some techniques, including microscopic hair comparison and bite mark analysis, have been widely discredited, while others remain the subject of continuing debate about their reliability and limitations. Cases involving arson science, ballistics and toolmark analysis, fingerprint evidence, microscopic hair comparison, bite mark analysis, and other techniques demonstrate the consequences: evidence accepted at trial may later be challenged or discredited. While many prisoners face legal difficulties raising junk science claims because of procedural hurdles, others have been able to do so because of new laws that recognize the scope of the problem.
Core Statistics:
- Nearly one third (65 of 203) of the people identified on DPI’s Exoneration List were wrongfully convicted and sentenced to death at least in part because of junk science or flawed forensic evidence.
- In a 2015 FBI review of 268 trials involving microscopic hair comparison testimony, examiners made erroneous statements in 257 cases, 96% of the cases reviewed.
- Defendants in at least 35 of the reviewed cases received death sentences; the FBI identified errors in 33 of those cases, or 94%. Nine of those defendants had already been executed, and five had died of other causes while on death row.
- The government initially identified nearly 3,000 cases in which FBI examiners may have submitted reports or testified using microscopic hair analysis. The FBI ultimately reviewed 23,547 cases before concluding its Microscopic Hair Comparison Analysis review.
- A 2023 National Institute of Justice study found that systematic errors across more than 30 forensic disciplines have contributed to wrongful convictions.
Key Facts:
- The term “junk science” gained prominence in criminal law in the late 1980s and early 1990s as courts and the scientific community increasingly questioned the reliability of longstanding forensic techniques. Advances in DNA testing further exposed weaknesses in a variety of disciplines by showing that methods once presented as objective science could produce inaccurate or misleading results.
- In Daubert v. Merrell Dow Pharmaceuticals (1993), the U.S. Supreme Court held that trial judges are to act as “gatekeepers,” assessing whether expert testimony is based on reliable scientific methods before it is presented to juries. A 2016 report by the President’s Council of Advisors on Science and Technology later identified significant concerns with the scientific validity of several commonly used forensic disciplines
- Texas was the first state to enact legislation to address junk science in the legal system. In 2013, the state created a mechanism allowing courts to reconsider convictions when advances in scientific knowledge undermine forensic evidence presented at trial. Several states have since adopted similar legislation, reflecting recognition that scientific understanding can evolve after a conviction.
- Texas’ statute does not guarantee relief. A July 2024 Texas Defender Service report found that the Texas law “systematically fails to provide relief to innocent people convicted based on false forensic evidence.”
Relevant and Current Cases:
- Virginia: Re-examination of DNA evidence
- The cases of eight executed men are being re-examined in Virginia as part of the state’s investigation into alleged mishandling of evidence by forensic scientist Mary Jane Burton.
- Texas: Charles Flores and investigative hypnosis.
- On June 15, 2026, the U.S. Supreme Court declined to review the case of Texas death-sentenced prisoner Charles Flores, whose conviction relied in part on testimony from a key prosecution witness who identified Mr. Flores only after being hypnotized by police.
- Mr. Flores sought relief under Texas’ junk science statute, arguing that advances in scientific knowledge undermined the reliability of evidence used to secure his conviction. Texas courts have refused to grant him relief.
- Louisiana: Jimmie Duncan and bite-mark evidence
- In June 2026, the Louisiana Supreme Court affirmed a lower court’s decision finding former death-row prisoner Jimmie Duncan factually innocent and vacating his conviction and death sentence. Mr. Duncan spent 27 years on death row before his release.
- Mr. Duncan was convicted based on faulty bite-mark evidence that Louisiana District Court Judge Alvin Sharp called “no longer valid” and “not scientifically defensible.” A previously undisclosed video, showing the forensic dentist using a mold of the defendant’s teeth to place marks on the victim’s body, was a turning point in the case.
- Chief Justice John Weimer wrote in his concurrence that the bite mark evidence used to prosecute Mr. Duncan was similar to “trial by water” tests used by witch-hunters in the 17th century, in which suspected witches were bound with rope and lowered into a body of water. “We now look back at those practices as asinine and absurd, since those who fell victim to those practices often did not survive, regardless of whether they were found guilty or innocent,” Judge Weimer wrote. “The bite mark evidence and the sexual abuse evidence used in the trial against the accused has proven to be similarly specious.”
Global Perspective:
Internationally, countries such as the United Kingdom, a nation that abolished capital punishment, have emphasized proactive regulation of forensic science through independent oversight bodies. England and Wales also maintain an independent Criminal Cases Review Commission to investigate potential wrongful convictions based on new evidence, including advances in forensic science.
