Bite mark evidence: the million-to-one claim in Chaney's trial
Bite mark evidence helped send Steven Mark Chaney to prison for a Dallas murder in 1987. A forensic dentist attributed an injury on John Sweek's arm to Chaney and supplied odds of one to a million against somebody else having made it. In September 2019, Texas Forensic Science Commission general counsel Lynn Garcia read that testimony into a congressional hearing. Chaney had served 28 years of a life sentence. Garcia also recorded a sharply different judicial outcome: the Texas Court of Criminal Appeals had declared him actually innocent on December 19, 2018.
The commission's intervention concerned the foundation beneath the comparison. Garcia testified that its members reviewed scientific literature and heard forensic dentists on both sides of the dispute. They recommended keeping bite mark comparisons out of criminal trials until sufficient data supported reliable, accurate results. The commission was an oversight body rather than a court, Garcia emphasized; judges retained responsibility for admitting evidence. Her testimony documents both the extraordinary numerical assurance offered at Chaney's trial and the later institutional demand for experimental support. The courtroom claim had preceded that support. Texas laboratories also had to disclose nonconformities, with the commission providing a forum outside adversarial proceedings for resolving them.
NIST's October 2022 scientific review announcement identified three unsupported premises behind bite mark evidence. The front-tooth patterns involved in biting had not been established as individually unique. Their accurate transfer to human skin was inconsistent. Examiners' ability to interpret the resulting patterns accurately enough to exclude or retain a possible source also lacked adequate support. These were separate weaknesses: a person's dental features, the injury they might leave, and the subsequent interpretation could each introduce uncertainty. NIST's April 2023 standards bulletin subsequently announced the finalized review, describing the changes after public comment as minor.
Skin adds specific difficulties. NIST described distortion from elasticity and movement during a bite, followed by changes from swelling and healing. It also noted disagreement among examiners about whether some photographed injuries were bites at all. The professional guidelines NIST described permitted exclusion, non-exclusion, or an inconclusive finding. Non-exclusion meant that the person's teeth could have caused the injury; it did not identify that person as its source. NIST's review concerned injuries in skin and did not evaluate identification of human remains through dental records. No population studies had established the frequency of distinguishing tooth features. The review considered more than 400 publications, including professional standards and conference presentations.
Chaney's legal team later traced how the numerical claim had developed. The Innocence Project's case history describes an initial estimate in the thousands to one, a later figure of 100,000 to one, and the million-to-one testimony ultimately delivered to the jury. In a sworn affidavit submitted with the 2015 habeas petition, dentist Jim Hales withdrew that testimony as scientifically unsound. Another prosecution dentist, Homer Campbell, had also attributed the mark to Chaney with dental certainty. The defense dentist had accepted that Chaney could not be excluded while declining to identify him with certainty. Those positions were substantially different even before the later research review.
The reinvestigation extended beyond bite mark evidence. According to the Innocence Project, newly disclosed files included earlier negative blood examinations of Chaney's shoes. DNA testing excluded him from male profiles found on or beneath Sally Sweek's fingernails, and he was excluded as the source of hairs in her hand. Dallas prosecutors supported relief following their own reinvestigation. Judge Dominique Collins ordered the conviction vacated in October 2015, and Chaney left prison. The appellate ruling followed in December 2018; Collins dismissed the case on January 16, 2019.
In its contemporaneous December 2018 announcement, the Innocence Project described the decision as relief under Texas's change-in-science law as well as a declaration of actual innocence. The organization had represented Chaney and sought the commission's investigation, so its announcement also expressed its own litigation position. It reported that the commission's six-month examination had prompted a recommended moratorium and an audit of Texas convictions involving the technique. The announcement credited prosecutors Patricia Cummings and Cynthia Garza with spending hundreds of hours on the reinvestigation. The legal remedy addressed scientific testimony whose standing had changed since trial, allowing that change to matter after an initial conviction and appeal.
The same announcement distinguished Chaney's conviction from the wider scientific assessment. It pointed to the 2009 National Academy of Sciences report and the 2016 presidential forensic-science review, which challenged reliable identification from skin injuries. Its demand for eliminating the technique was advocacy; Garcia's congressional testimony documented the commission's conditional recommendation, and NIST identified the unsupported scientific premises. Together these records locate the central failure in bite mark evidence: strong individual attribution and numerical certainty had entered a murder prosecution without the scientific foundation later investigators required.
- U.S. House Committee on Science, Raising the Bar: Progress and Future Needs in Forensic Science, Lynn Garcia testimony, September 10, 2019 — www.govinfo.gov
- NIST, Forensic Bitemark Analysis Not Supported by Sufficient Data, October 11, 2022 — www.nist.gov
- Innocence Project, Steven Mark Chaney case history — innocenceproject.org
- Innocence Project, December 19, 2018 announcement of the Chaney ruling — innocenceproject.org
- NIST, OSAC Standards Bulletin, April 2023, announcement of the finalized bitemark review — www.nist.gov
- Innocence Project, Andrew Z. Giacalone, Bite Mark Testimony Comes under Fire Yet Again, October 21, 2015: Chris Fabricant on unsupported assumptions — innocenceproject.org