This week, the National Institute of Standards and Technology (NIST) released a long-awaited report on the use of bitemark analysis. This is the disipline in which a specialist matches an apparent bite mark on human skin and to the teeth of the person who did the biting. The field has a horrendous track record: More than two dozen people arrested or convicted with bitemark evidence have since been exonerated.

The NIST report has been a long time coming. In its landmark 2009 report on forensic evidence, the National Academy of Sciences singled out bitemark analysis as a forensic specialty with little to know scientific underpinnings, and called for further study. This report is the answer to the NAS report, and it's a thorough rebuke of all things bitemark analysis. 

The entire 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 we do have 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. Members of the NIST panel will present a webinar on their findings later this month.

The NIST report is now the fourth major scientific body to find no scientific validity to support these central tenets of bitemark analysis. In addition to the 2009 NAS 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. But that only makes the criminal justice system's refusal to course-correct here all the more damning. At worst, it would require the re-opening and reinvestigation of a few hundred cases. Yet even those relatively limited consequences are apparently too much for the courts to bear. 

In fact, it would be difficult to come up with a starker, clearer example of the incongruity between the courts' and the scientific community's conception of expertise. Critics began sounding alarm 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 grossly unreliable. Meanwhile, remarkably, 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.

If the criminal legal system prioritized justice, we'd have long ago seen a thorough review of every bitemark conviction in the country — if not after the first series of DNA exonerations of bitemark convictions, then certainly after the NAS report cast doubt on the entire discipline. There would have been an urgent, systemic effort to identify other people who may have been wrongly convicted. Instead 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. Nearly every time, the courts have subsequently ruled for prosecutors. 

Just last year, a judge in Alabama upheld the 1985 conviction of Charles McCrory based almost entirely on bite mark evidence, despite the fact that even the state's expert has since renounced his own testimony. Prosecutors argued that the jury could have simply substituted its own bite mark analysis for that of the expert, a gobsmacking argument that would seem to refute the entire reason for bitemark analysis -- that it's a specialty that can only be performed by highly-trained experts. 

The judge adopted the prosecution's argument verbatim. 

But this isn't just a red state problem. Two of the leading evangelists for bite mark analysis today are Melissa Mourges and Robert Ferrari, both longtime prosecutors in the Manhattan District Attorney's Office. Mourges's impassioned defense of bite mark evidence has even extended to launching personal attacks against the scientists who have criticized it. Her brazen mischaracterization of the 2009 NAS report in a New York case has used as a template by prosecutors in other cases, including in a 2017 Pennsylvania case. That prosecutor won, too.

Our system does sometimes cop to mistakes in individual cases. But if admitting error in a specific case risks exposing more structural failures, the courts tend to double down. The Alabama case, and bitemark cases more generally, are a striking example of the depths to which the system will plunge to avoid admitting to systemic flaws — in this case, the defective process by which courts determine the validity of expert testimony.

Because the criminal justice system was created by and is operated by human beings, mistakes will be inevitable. The courts' legitimacy lies in their ability to concede and correct them. Tragically, judges and prosecutors seem to have concluded that real legitimacy lies in pretending the biggest, most consequential mistakes never happened.