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Why is forensic bitemark identification likely to be abolished as a form of trial evidence?

The public holds exaggerated views of the quality of the scientific foundations of a surprising number of forensics sciences, as well as of the courts’ scrutiny of that evidence. The most significant of the weaknesses were made plain in a report by the US National Academy of Sciences (NAS), which concluded, “The bottom line is simple: In a number of forensic science disciplines, forensic science professionals have yet to establish either the validity of their approach or the accuracy of their conclusions… Much forensic evidence including, for example, bite marks and firearm and tool mark identifications is introduced in criminal trials without any meaningful scientific validation, determination of error rates, or reliability testing.” Studies of wrongful convictions based on DNA exonerations have found forensic errors and exaggerations to be second only to eyewitness errors.

Several forensic techniques that had long been welcomed into American courts are now dead, found by scientific review committees to lack sufficient validity. The eulogy for voice-prints was given by the NAS in 1979, following which the FBI ceased offering such experts, and the discipline slid into decline. More recently, comparative bullet lead analysis met the same fate. And, over a continuing period, numerous “indicators” of arson have been determined to lack validity and have been laid to rest.

A likely candidate to join those techniques in the cemetery of departed forensic sciences is forensic odontology – which compares bitemarks (usually found in the flesh of crime victims) with the dentition of suspects. Forensic dentists claim the ability to associate a bitemark to the one set of teeth in the world that could have produced the crime scene bitemark. However, no sound basis exists for believing they can perform such a feat. As the NAS Report observed, “the scientific basis is insufficient to conclude that bite mark comparisons can result in a conclusive match.”

Remarkably, no American court has excluded bitemark evidence based on its failure to meet the legal standard for admission of expert testimony. Only in rare instances have judges expressed doubts about its trustworthiness. This is changing. In a series of high-profile cases, often involving DNA exonerations, bitemark identifications have been exposed as erroneous. The Texas Commission on Forensic Science has called for a “moratorium” on the use of bitemark testimony in court and is auditing old cases that had involved the use bitemark evidence. Most recently, the US President’s Council of Advisers on Science and Technology recommended that, in the absence of sound research supporting the reliability and validity of bitemark identification evidence, prosecutors not offer it and, if offered, that courts not admit it.

Courtroom by David Mark. CC0 Public Domain via Pixabay.

Here are the major scientific deficiencies of bitemark identification expert evidence:

Source. Adult dentition consists of 32 teeth, each with 5 anatomic surfaces, creating 160 dental surfaces that could contain identifying characteristics. But a typical crime scene bitemark offers only a fraction of that information – only 4 to 8 teeth are involved in biting. Compared to other types of forensic evidence, less information is available to work with.

Substrate. Bitemarks typically occur during struggles, where skin is stretched and contorted. When the skin returns to its normal shape, the impression of the biter’s dentition is distorted to an unknown extent. Even without such distortion, skin is a poor substrate for registration of tooth impressions.

Methods of Comparison. When a forensic dentist compares a bitemark with a suspect’s dentition, numerous techniques may be used, including drawing images by hand. “The issue of the multiple methods of bitemark analysis continues to thwart any attempts to standardize procedures to any sort of ‘gold standard.’”

Evaluation of an Inclusion. If the decision reached by the examiner is inclusion of the suspected source, the next step is to evaluate the meaning of that inclusion. Its probativeness depends upon how many others in the population could also have produced markings that are equally similar in appearance to the crime scene marks. Virtually no data exist on the population distribution of dentition attributes.

Uniqueness. The conventional solution to the problem of assessing the meaning of an inclusion has been to assume uniqueness. If no two biters could be mistaken for each other, an inclusion would lead directly to the conclusion that the source had been found. But, as the NAS Report notes, there is “no evidence of an existing scientific basis for identifying an individual to the exclusion of all others.”

Examiner Reliability. The American Board of Forensic Odontology recently conducted a reliability study of the judgments of experienced, board-certified forensic dentists making basic decisions about bitemarks. The researchers selected 100 photographs of suspected bitemark injuries from actual cases. These were examined by 38 ABFO-certified odontologists. The examiners were asked three questions about each of the 100 injuries. (1) Is the injury a bitemark? (2) Is it a human bitemark? (3) Does the bite mark have distinctive features to enable individualization? No one can know which answers were correct; all we can know is the extent to which examiners agreed or disagreed with each other. Taking all three questions together, for just under half of the cases, not even 50% of the examiners agreed on the same trio of responses. For only 14 of the 100 cases did at least 80% of the examiners agree on the trio of responses. If there is little agreement (low reliability) among examiners, there can be little validity in their judgments.

Examiner Accuracy. Over approximately four decades in which forensic dentists have been testifying and claiming the ability to identify the individuals who inflicted bitemarks, only a handful of studies were carried out to assess their accuracy. Those tests presented practitioners with “questioned” bitemarks that are placed in materials that are more stable than living human flesh. The task is to try to match questioned bitemarks to a possible source (a bitemark lineup). Error rates in these studies are unimpressive and sometimes dreadful.

Lack of Standards. No standard exists for the type, quality, and number of characteristics required to conclude when a bitemark has reached a threshold of evidentiary value. In comparing images of questioned and known bites, examiners are left to their own subjective judgment.

Under existing law, it is hard to understand how forensic bitemark expert evidence continues to be admissible in court. The probability that it soon will be excluded seems to be increasing rapidly.

Featured image credit: Barbed Wire by Erika Wittlieb. CC0 Public Domain via Pixabay.

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