The National Academy of Sciences Slams Bite Mark Analysis


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.

NEXT: At Return to the Gulag—Jon Utley's Search for His Father

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  1. Folks may be interested to know that the NPR show “Talk of the Nation: Science Friday” did a ~30 minute segment yesterday on this NAS report and the state of forensic science:

    **Report Finds Forensic Evidence Lacking**
    A new report from the National Academy of Sciences says much of what’s commonly called “forensic sciences” doesn’t meet scientific standards. Experts say the country’s forensics methods and systems – from fingerprint identification to bite-mark analysis – need an overhaul.

  2. Any expert witness that presents this evidence in court should allow the defendent to bite them on the arm and show the similarity to the evidence in question. Unless the defendent is a vampire, in which case they may like it.

  3. I’ve long been convinced that bite mark analysis is, scientifically, only a small step above phrenology.*

    The real bombshell in the report is this

    There is some evidence that fingerprints are unique to each person, and it is plausible that careful analysis could accurately discern whether two prints have a common source, the report says. However, claims that these analyses have zero-error rates are not plausible; uniqueness does not guarantee that two individuals’ prints are always sufficiently different that they could not be confused, for example. Studies should accumulate data on how much a person’s fingerprints vary from impression to impression, as well as the degree to which fingerprints vary across a population. With this kind of research, examiners could begin to attach confidence limits to conclusions about whether a print is linked to a particular person.

    I’ve read others cast doubts on the reliability of fingerprint evidence (Sketic magazine?) which runs contrary to the conventional wisdom we’ve all been exposed to since early childhood. That the NAS couldn’t unearth any studies that conclusively support fingerprint evidentiary claims of reliability is unnerving to say the least.

    I strongly suspect individual firearm identification extrapolated from recovered bullets is in even worse shape. I’m not going to pay to read the whole report though. I will be on the lookout for a decent summary of it.

    * Full disclosure – The NAS rejected my application for membership. It seems the elitist group requires actual educational attainments and professional accomplishments in the sciences in order to be considered. 😉

  4. Sparky,
    Thanks for the info and link.

  5. 2/10.

    Extra point awarded for posting without a timestampe (?!?), but major deductions for failure to use the term Kochtopus.

  6. major deductions for failure to use the term Kochtopus

    I was trolling as lefty #.Lefties don’t say “Kochtopus”.
    Ultra-righty # might use “Kochtopus”.
    The link is a charge by Playboy‘s blog that the Santelli rant is a RightWing plot funded and orchestrated by “the Kochtopus” including it’s WingNut tentacle Reason.

  7. Next to Girls of the Ivy League, that’s the best thing I’ve ever read in Playboy. Thanks!

  8. So did you jerk off more to “Girls of the Ivy League” or the Koch blogpost, LoneJerkOff?

  9. I’m disappointed, Radley. You could have gone with “Taking A Bite Out of Crime Analysis” or “NAS Chews Out Bite Mark Analysis” or even “NAS to Forensics: Bite Me.”

    Sigh. So many wasted possibilities.

  10. I’m glad to see this again. I recall hearing that this kind of analysis is seriously lacking quite a few years ago, but haven’t seen it since.

    Regarding the not-so-perfectness fingerprint analysis, DNA is not nearly as cut & dry as people think, either. The probability calculations assume that the technician has correctly identified matches, which can be very subjective.

  11. The probability calculations assume that the technician has correctly identified matches, which can be very subjective.

    This is true. And the odds against an accidental positive misidentification of any specific individual are beyond astronomical, so any “false positive” is willfully falsified. But no unmotivated party checks the huge majority of DNA results, because they’re so science.

    The number of posthumous exonerations in a few decades would be staggering, if convicting evidence were preserved. But it mostly isn’t. So hey.

  12. two wonderful highlights for me

    Sparky | February 28, 2009, 7:45pm | #

    So did you jerk off more to “Girls of the Ivy League” or the Koch blogpost, LoneJerkOff?

    i feel like i have contributed to the history what is now a professional sport of kicking lonewacko in the nuts every time he posts. I am proud of all of you.

    TallDave | February 28, 2009, 8:13pm | #

    I’m disappointed, Radley. You could have gone with “Taking A Bite Out of Crime Analysis” or “NAS Chews Out Bite Mark Analysis” or even “NAS to Forensics: Bite Me.”

    Sigh. So many wasted possibilities.

    Every now and then TallDave fails to say something ridiculous about how Iraq is a phenomenal strategic success, and actually makes me laugh and think, “maybe he’s not a *total* schmuck”

  13. The number of posthumous exonerations

    I first read this as “the number of posthumous exhumations” and I was thinking ‘I certainly hope so!’

  14. Missing in this discussion: the failure of defense attorneys to bring this up. Daubert is a federal standard but many states follow it or something similar. If the science of bite mark forensics and other alleged identifiers is scientifically bad, they should object and raise that as an issue. If it hasnt been done in so many cases, or done effectively, there is where a systemic problem also exists.

  15. hogan, lawyers know the law more than the science the labs use to prosecute their clients. Until the NAS article, what leg did they have to stand on? An objection would sound like a failed argument to any jury prior to the NAS claim which is fairly recent.

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