Science

The National Academy of Sciences Slams Bite Mark Analysis

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A couple of weeks ago, I wrote about a forthcoming study from the National Academy of Sciences that's extremely critical of how forensic science is used in the courtroom. The study came out a little over a week ago. In light of my story about possible forensic bite mark fraud by Mississippi forensic experts Steven Hayne and Michael West, it seems particularly relevant to look at what the NAS study had to say about bite mark evidence.

The first mentions of bite mark analysis, or forensic odontology, occur in a couple of footnotes. The first says…

…forensic odontology might not be sufficiently grounded in science to be admissible under Daubert, but this discipline might be able to reliably exclude a suspect, thereby enabling law enforcement to focus its efforts on other suspects. And forensic science methods that do not meet the standards of admissible evidence might still offer leads to advance an investigation.

Daubert refers to a trio of Supreme Court cases from the early 1990s that establish the admissibility standards of expert testimony in the courtroom. The NAS study says here that despite the fact that bite mark evidence has been introduced in hundreds, likely thousands of cases since Daubert, bite mark analysis, while possibly useful for investigative purposes, may not be steeped enough in science for consideration in a courtroom. Indeed, the report next chides the courts for not even performing a proper Daubert evaluation in too many bite mark cases.

Much forensic evidence—including, for example, bite marks and firearm and toolmark identifications—is introduced in criminal trials without any meaningful scientific validation, determination of error rates, or reliability testing to explain the limits of the discipline.

In the section directly addressing bite mark analysis, the study found that there is no scientific research showing any particular uniqueness to human dentition (put another way, there's no basis for saying that the impressions our teeth make is unique to us the way our fingerprints or DNA are). Moreover, even if such uniqueness could be established, it's even less clear that said uniqueness would be transferred when human teeth bite down on human skin. The study finds that due to the elasticity of human skin, swelling and healing, and unevenness of the skin's surface, the marks made from a bite can morph over time. And the study is even more skeptical of analyses done from photographs of bite marks, given that photographs allow for further distortion of the marks.

The study's authors are also concerned that there's no scientific research showing that the various methods used by bite mark experts are reproducible, both "between experts and with the same expert over time." This is one of the pillars of scientific inquiry—that a scientist's results can be reproduced by other scientists. There are several studies in which bite mark experts plying their craft in controlled laboratory tests have shown "widely differing results and a high percentage of false positive matches."

The authors add:

As with other "experience-based" forensic methods, forensic odontology suffers from the potential for large bias among bite mark experts in evaluating a specific bite mark in cases in
which police agencies provide the suspects for comparison and a limited number of models from  which to choose from in comparing the evidence. Bite marks often are associated with highly sensationalized and prejudicial cases, and there can be a great deal of pressure on the examining expert to match a bitemark to a suspect. Blind comparisons and the use of a second expert are not widely used.

The NAS study's bite mark section concludes:

Although the majority of forensic odontologists are satisfied that bite marks can demonstrate sufficient detail for positive identification, no scientific studies support this assessment, and no large population studies have been conducted. In numerous instances, experts diverge widely in their evaluations of the same bite mark evidence.

Bite mark testimony has been criticized basically on the same grounds as testimony by questioned document examiners and microscopic hair examiners. The committee received no evidence of an existing scientific basis for identifying an individual to the exclusion of all others. That same finding was reported in a 2001 review, which "revealed a lack of valid evidence to support many of the assumptions made by forensic dentists during bite mark comparisons," which has led to questioning of the value and scientific objectivity of such evidence.

The authors say more research is needed to see if bite mark analysis might have some probative value, for example to exclude suspects. But there's no reliable research to back up claims that there's any real science behind the idea that an expert can positively match a bite mark left on skin to an individual person.

That ought to be cause for pretty grave concern in any case where an expert claimed he could positively link a bite mark to a suspect, but it should cause particular concern where bite marks were the only evidence linking the suspect to the crime.