Despite the popularity of cop shows about investigators bringing criminals to justice based on a few fingernail clippings and a dropped tissue, the track record of forensic evidence is spotty at best. Critics, including Reason journalists, have shown that too much crime-stopper "science" resembles tea-leaf reading more than it does the efforts of Sherlock Holmes.
Now a federal agency says that bitemark analysis, something of a poster child for bad forensic technique, is every bit as sketchy as skeptics claim.
"Forensic bitemark analysis lacks a sufficient scientific foundation because the three key premises of the field are not supported by the data," finds a draft report from the National Institute of Standards and Technology (NIST). "First, human anterior dental patterns have not been shown to be unique at the individual level. Second, those patterns are not accurately transferred to human skin consistently. Third, it has not been shown that defining characteristics of those patterns can be accurately analyzed to exclude or not exclude individuals as the source of a bitemark."
There's a lot more in the report, which is currently in its comment period and so isn't yet finalized. It's worth noting this 2022 document is a response to a 13-year-old call for a stronger scientific basis for the proliferating use of forensic evidence.
"NIST scientific foundation reviews fill a need identified in a landmark 2009 study by the National Academies of Sciences, Engineering, and Medicine, which called for research to address issues of accuracy, reliability and validity in many forensic science disciplines, including bitemark analysis," acknowledges NIST.
That was after journalist Radley Balko, then with Reason, had pointed out some of the glaring flaws in forensic science in general, and bitemark analysis in particular.
"He claims to have perfected a method of identifying bite marks using laser light and orange goggles that he modestly calls 'the West Phenomenon,'" Balko wrote in 2007 of Mississippi dentist Michael West. "He has said his error rate in bite mark analysis is 'something less' than the error rate of 'my savior, Jesus Christ' and has compared his bite mark virtuosity with the musical talent of Itzhak Perlman."
Balko subsequently literally wrote the book about West, fellow practitioner Dr. Steven Hayne, and the injustices resulting from their forensic testimony. But even with NIST acknowledging, at long last, that "the ability of bitemark analysis to accurately exclude or not exclude individuals as a source of the mark is not supported," he sees little hope for reform.
"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," Balko writes. "Tragically, judges and prosecutors seem to have concluded that real legitimacy lies in pretending the biggest, most consequential mistakes never happened."
Maybe not, but it's worth highlighting the extent to which the NIST report debunks bitemark analysis. The report points out that bitemark analysis relies on the assumptions that: teeth marks are unique; that they reliably transfer to surfaces such as skin; and that the marks can then be analyzed and linked to specific individuals.
First of all, it's not at all clear that bites leave distinct patterns. "Bitemark patterns typically only represent the anterior teeth" (those in the front of the mouth) and the marks they leave can vary depending on injuries, breakages, or obstructions. That leaves limited information with which to work even before we get to the contradictory evidence available about the individuality of bites from one mouth to the next.
Second, skin is the surface most often analyzed for bitemarks, but it's malleable and doesn't reliably take teeth marks to begin with. "In addition, human skin can change the appearance of a bitemark over time depending on the rate and amount of swelling at the site, healing, and skin elasticity." As a result, "human skin as a dependable material for bitemarks is a key area of dispute in the field."
Finally, linking bitemarks that may or may not be unique, left on elastic surfaces that swell and heal, to people is fraught with uncertainty. "Multiple studies have demonstrated a widespread lack of agreement on conclusions reached with bitemark data, including those relating to whether the mark was indeed a bitemark, features present, and inconsistency in techniques used to analyze bitemarks from one case to the next."
That's right. Scientists don't always agree they're examining a bitemark, let alone on who left it.
The NIST report might or might not inspire some humility in the criminal justice system. But it should offer ammunition to defendants against whom bitemarks are included in the prosecution's evidence.
Unfortunately, as that 2009 NAS call for better scientific support for forensic science suggests, the problem doesn't stop with bitemarks. Reason has documented the unreliability of drug-sniffing dogs, flawed drug tests, shaken-baby junk science, and the sometimes dishonest testimony of technicians called to make the state's case. The unreliability of much of this evidence isn't a recent revelation.
"In September the President's Council of Advisers on Science and Technology (PCAST) released a report finding 'a dismaying frequency of instances of use of forensic evidence'—such as analyses of hair, bite marks, and shoe prints—'that do not pass an objective test of scientific validity,'" C.J. Ciaramella wrote for Reason in 2016. "This is not just a theoretical problem. Last year, the FBI admitted that nearly every one of the experts at its microscopic hair analysis lab had given scientifically invalid testimony. The breaches affected almost 270 cases. Of those, 32 defendants were sentenced to death, and 14 were executed or died in prison."
So, the NIST report on the failings of bitemark analysis provides extra backing for what even the White House admitted two administrations ago: those cop shows about super-accurate science linking criminals to their foul deeds are more science fiction than whodunnit. As it turns out, it takes a lot more evidence to end the use of bad forensic techniques than it does to throw people in prison or put them on death row.
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