Radley Balko April 8, 2015
A bite mark matching advocacy group just conducted a study that discredits bite mark evidence
In February, I posted a four-part series on the forensic speciality of bite mark analysis. The series looked at the history of the field, how it came to be accepted by the courts as scientific evidence despite the lack of any real scientific research to support its basic assumptions, the innocent people who have been convicted based on bite mark analysis and how the bite mark matchers, advocacy groups like the American Board of Forensic Odontology and their supporters have waged aggressive, sometimes highly personal campaigns to undermine the credibility of people who have raised concerns about all of this.
The series ran during the annual American Academy of Forensic Sciences convention in Orlando, Florida. That conference included a presentation by Adam Freeman, who sits on the executive board of the ABFO, and Iain Pretty, who is not a member of the ABFO, has been critical of bite mark analysis and chairs the AAFS committee on forensic odontology.* Freeman and Pretty were to present the results of a study they had designed with David Senn, another ABFO member and a proponent of bite mark analysis.**
Senn in fact was the main witness for New York County Assistant District Attorney Melissa Mourges during a 2013 evidentiary hearing on the scientific validity of bite mark analysis in State v. Dean. That hearing was the first to assess the science behind bite mark matching since the field came under fire in a landmark 2009 report by the National Academy of Sciences. Ultimately, Senn and Mourges prevailed. Judge Maxwell Wiley ruled that the evidence could be admitted at Clarence Dean’s trial. In fact, to date, every court to rule on the admissibility of bite mark analysis has allowed it to be used as evidence. This, despite an ever increasing number of wrongful convictions, wrongful arrests, and lack of scientific research to support the field, and a new body of research suggesting that its core assumptions are false.
All of this makes the presentation by Pretty and Freeman particularly interesting. In response to mounting criticism, last year the ABFO released a “decision tree” for bite mark specialists to follow when performing their analysis. The “tree” is basically a flow chart. It begins by asking if there is sufficient evidence to know whether or not a suspicious mark is a human bite. It then asks whether it is in fact a bite, then what distinguish characteristics are noticeable in the bite, and so on. But the problem with bite mark analysis was never the lack of a flow chart. The problem is that there has never been any real scientific research to support its two main underlying premises — that human dentition is unique, and that human skin is capable of registering and recording that uniqueness in a useful way. And the research that has been done strongly suggests those two premises are not true. The flow chart was just adding a series of procedures to a method of analysis that is entirely subjective, and that lacks basic scientific quantifiers like probability and margin for error.
Yet the ABFO wanted to show that its flow chart worked. So last year, the organization put together an exam to prove its effectiveness. Pretty and Freeman, with consultation from Senn and others within the organization, gave 39 ABFO-certified bite mark analysts photos of 100 bite marks, then asked them to answer three preliminary questions, all based on the decision tree chart. The average analyst who participated in the study had 20 years experience as a forensic odontologist. Here are the three questions they were asked:
1) Is there sufficient evidence in the presented materials to render an opinion on whether the patterned injury is a human bite mark?
2) Is it a human bite mark, not a human bite mark, or suggestive of a human bite mark?
3) Does the bite mark have distinct, identifiable arches and individual tooth marks?
That last question is asking if, once the analyst has determine that the mark is a human bite, the mark contains enough distinguishing features to be of value as evidence.
Read more for the test results and the later ‘damage control’ by these a few US forensic dentists. A bitemark group discredits itself in public