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An Argument Without Teeth: The Flawed Science of Bite Mark Analysis

by Eike Blohm, MD

A review published by the National Institute of Standards and Technology (“NIST”) found that forensic bite mark analysis lacks sufficient scientific foundation.

Adult humans have 32 teeth but only the foremost dozen leave marks when one person bites another. Analyses of the patterns of injury are routinely used in courts as evidence similar to fingerprints. Forensic dentists opine as expert witnesses whether a defendant’s dentition could have produced the bite marks found on the victim’s skin. Juries determine the defendant’s guilt with results from bite mark analysis in mind.

So did the jury that heard the case against Robert Stinson, accused of the 1984 murder of Ione Cychosz in Milwaukee, Wisconsin. Her body featured eight bite marks from which a forensic dentist determined that her assailant was missing the right upper lateral incisor, the second tooth from the middle. When detectives interviewed Stinson, they noted he was missing the right upper central incisor (middle tooth) – close enough. There were no eyewitnesses, no fingerprints, and no motive that linked Stinson to the murder, but the jury found him guilty based on expert testimony that his dentition was “identical” to the bite mark. He was exonerated 23 years later based on DNA analysis.

Bite mark analysis has always been unreliable. At a 1999 workshop of the American Board of Forensic Odontology – the accrediting body for self-styled experts in bite mark analysis – experts were tasked with matching four bite marks to seven dental models. About 63% of attendants identified the wrong dentition for a given wound.

In a 2015 study, 39 experts were shown 100 photographs of bite marks. They were asked to determine if the bite was from a human or animal and if there were sufficient data points to use the bite mark as evidence in court. Astonishingly for a discipline that holds itself out as a science, in only 8% of photographs could experts reach 90% consensus.

A 2016 study found similar results. When experts were shown bite marks and then the same bite marks eight weeks later, not only did they disagree with one another but also their own prior assessments.

Because of these inherent problems, the guidelines from the American Board of Forensic Odontology advise experts to issue findings limited to “excludes” or “does not exclude” defendant or “inconclusive.”

The reason why bite mark analysis is unreliable is multifactorial. When a bite mark is taken out of a chocolate bar, it leaves detailed imprints of the dentition. Spacing between teeth can be measured, and the angulation of teeth estimated. But skin is a poor medium for capturing those details. Unlike chocolate, skin bunches up or stretches when bit, distorting the dental print. The victim might try to pull away, adding relative lateral motion between skin and teeth that converts a puncture or crush injury into a lacerating mechanism.

After the bite has occurred, the injured tissue will swell and increase the spacing between individual data points. Medical treatment such as injection of local anesthetics, wound exploration for underlying tendon or vessel injuries, or removal of both fragments further distort the bite mark. If a victim does not contact police immediately, partial healing of the bite obscures the appearance of the original wound.

Those are just the technical problems. The deeper issue with bite mark analysis is that it is constructed on several flawed premises. It assumes that human dentition patterns are unique – but that has never been established. When NIST conducted a systematic review of all scientific literature in English about bite mark analysis, it found no data to support the assertion that individuals have unique dental patterns.

Even if dentition were unique, none of the 400 scientific publications reviewed established that this pattern is reliably transferred to skin. In fact, a 2011 study by Mary Bush et al. in Forensic Science International found the opposite: pattern transfer is highly unreliable.

Lastly, there exists no standard for the number, type, and quality of data points in a bite mark analysis that establishes when a bite wound crosses the threshold of being of evidentiary value.

Due to the problem outlined in the NIST review – as well as by a 2009 review by the National Academy of Sciences with similar findings – bite mark analysis shouldn’t satisfy Federal Rule of Evidence 702. This rule provides that expert witness testimony must be the product of “reliable principles and methods,” which can’t be said about bite mark analysis. Yet courts continue to allow bite mark analysis as evidence, mostly citing precedence that prior cases also allowed it. That’s the equivalent of allowing current injustices to occur because previous courts have permitted the same injustice to occur. This makes removal of this pseudoscience from the courtroom an arduous and painful task – akin to pulling teeth.

Sources: NIST.gov, psychologytoday.com

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