Pennsylvania Court Refuses to Hear Criticisms of Bite Mark Evidence

By Matt Kellner

For at least one courtroom, in Altoona, Pennsylvania, scientific understanding never changes.

In an opinion delivered last week in the capital murder case of Paul Aaron Ross, Judge Jolene Grubb Kopriva of the Blair County Court of Common Pleas refused Ross’s request to hold a pretrial hearing on the validity of bite mark comparison evidence that the prosecution intends to use against him. Although Ross—who is represented by Pennsylvania attorneys Thomas Dickey and Thomas Hooper with the assistance of the Innocence Project’s Strategic Litigation Department—presented ample and uncontroverted documentation outlining the fundamental unreliability of bite mark comparisons, the court ruled that judicial scrutiny applies only to “novel methodologies.” As a result, the court reasoned that problematic “existing scientific field[s],” such as the bite mark comparison evidence questioned by Ross, fall outside of the scope of the court’s review.

Although bite mark evidence is fairly obscure, the technique has garnered widespread and consistent criticism for its lack of scientific reliability. Study after study has undermined the basic assumptions underlying the technique and demonstrated that its practitioners have a “disturbingly high false-positive error rate.” In the courtroom, that alarming error rate has manifested itself through at least 28 wrongful convictions and indictments, and these are only those that we know of.

On the basis of this overwhelming scientific evidence, Ross, who faces a retrial in connection with a 2004 homicide, petitioned the court in Blair County to prevent the Commonwealth from introducing this unreliable evidence, or at minimum to hold a hearing on the issue.


During Ross’s first trial—in which the Pennsylvania Supreme Court found serious constitutional flaws because Ross’s attorney wasn’t given enough time to prepare, among other errors—a forensic dentist testified that his teeth, unlike those of other suspects, were “very highly consistent” with a purported bite mark on the victim. Now, at Ross’s upcoming trial, the Blair County District Attorney intends to have the same dentist, as well as another so-called expert, testify to the same conclusions, despite significant scientific advances discrediting bite mark analysis since Ross’s first trial.

In particular, in the decade since that testimony was initially offered, three independent, prestigious scientific bodies have reviewed and ultimately rejected the reliability of bite marks. In 2009, the National Academy of Sciences issued a landmark report, which found that “the scientific basis is insufficient to conclude that bite mark comparisons can result in a conclusive match.”

The NAS’s concerns were echoed in two further reports published in 2016, from the Texas Forensic Science Commission (TFSC) and the President’s Council of Advisors on Science and Technology (PCAST). The TFSC both recommended a moratorium on the use of bite mark evidence in criminal cases across the state of Texas and has begun an audit of all Texas convictions where bite mark evidence was used. For its part, the PCAST report determined that the discipline was so far from reaching standards of foundational validity that researchers would be unwise to invest in trying to prove its reliability.

When confronted with these damning critiques, the Altoona court deemed them irrelevant, focusing only and improperly on the opinions of the very individuals whose livelihoods depend on the continued use of bite marks: forensic dentists.

Under Pennsylvania’s legal standard for admitting expert evidence, courts must consider whether a technique is generally accepted by the “relevant scientific community.” Though that community certainly consists of more than just bite mark practitioners, the court defined it as narrowly as possible, to exclude the research scientists, academics, and scientific organizations that have found bite marks fundamentally unreliable, and thus to sidestep the vast corpus of peer-reviewed research criticizing the technique.

Such willful scientific illiteracy among the judiciary flies in the face of the Supreme Court’s mandate over 30 years ago that capital cases must “aspire to a heightened standard of reliability.” Bite mark evidence has already landed at least two innocent people on death row. That courts are still unwilling to even hear how unreliable the discipline is should give us pause, especially so when, as with Ross’s case, liberty and life are at stake.

1 Comment

  1. Gentlemen:
    Have Mr. Ross’s defense attorneys appealed this grotesque ruling to a higher court?? Bite-mark evidence should NOT be allowed into Mr. Ross’s trial. Otherwise, he will likely be CONVICTED of capital murder on FALSE evidence. There was such a case in Phoenix, Arizona in the 1990’s, where a man WAS convicted of capital murder and sentenced to death, based mainly on bite-mark evidence. He spent ten years on death row, and came very close to execution. With his time running out, DNA evidence was appealed to the 9th Federal Circuit Court of Appeals, and his conviction was thrown out. Since bite-mark “evidence” was the main “evidence” against him, his entire case was vacated and he was exonerated of all charges and released. WHY can’t this case in Arizona, involving a false, bite-mark capital murder conviction, be used as a valid, legal precedent for Mr. Ross to be acquitted of his capital murder charges??

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