The National Institute of Standards and Technology (NIST) has released a long-awaited report on analyzing bitemarks, the discipline in which a specialist matches an apparent bite mark on human skin to the teeth of the person who did the biting. More than two dozen people arrested or convicted with bitemark evidence have since been exonerated, writes Radley Balko in Substack. The specialty of bitemark comparison rests on three core assumptions. The first is that each person's teeth are structured and aligned in a way that causes them to leave unique bite marks. The second is that human skin is capable of recording and preserving those marks in a way that makes them distinguishable. The final assumption is that trained analysts are capable of analyzing marks to include or exclude someone as the person who left them. The NIST study found no scientific evidence to support any of these assumptions, and the evidence explicitly refutes two of them. Studies have consistently shown that human skin is incapable of recording and preserving the details of a bite, and competency tests have shown that not only are bitemark analysts bad at matching bites to human subjects, they often can't even agree on what is and isn't a human bite.
The NIST report is the fourth major scientific body to find no scientific validity to support these central tenets of bitemark analysis. On top of a 2009 National Academy of Sciences report, in 2016 the Texas Forensic Science Commission found the scientific support for bitemark comparison so lacking that it recommended a moratorium on its use in criminal cases. That same year, the President's Council of Advisors on Science and Technology also recommended a moratorium. Bitemark analysis has been used only in a small percentage of criminal convictions. At worst, re-examining bitemark cases would require the re-opening and reinvestigation of a few hundred cases. Critics began sounding alarms about the bitemark comparison almost as soon as it began to be accepted by the courts in the 1970s. Since then, nearly every outside, scientific, peer-reviewed analysis of the field has found it unreliable. Meanwhile, nearly every court to rule on its reliability has deemed it valid. Typically, the courts' "scientific" analysis has consisted of little more than listing all of the other courts that have previously found the field to be reliable. Now it's been left to public interest lawyers to persuade reluctant courts to reopen decades-old cases. Prosecutors continue to defend bitemark convictions, typically by asking courts to deny appeals for procedural reasons, by ignoring or playing down the scientific community’s conclusions, or by misrepresenting those conclusions entirely.