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What can experts say on bite mark comparison?

Richard Bassed, Pratik Tarvadi, expert opinion, forensic odontologist, bias, dental line up, cognitive bias, bitemark
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In terms of matching an individual body to an injury and human skin, the following conclusions apply. There is no real evidence to show that this is based in the scientific method. No reproducible experimental results. Too many unaccounted and unaccountable variables. And no positive clinical trials with real injuries. There is no avoiding the fact that human skin is a terrible impression material, that bite marks are always distorted in the skin due to the dynamic nature of biting. And there is no way to reproduce exactly body limb position and muscle tension present at the time of biting. There is also no avoidance of the fact that most people have very similar dentitions in terms of tooth placement and overall dimensions.
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Similar enough that to differentiate between them in distorted bite mark and skin is scientifically impossible. And finally, techniques for analysis are variable, often made up by individual practitioners and non-standard. The level of concordance between experts is extremely low. The entire premise of being able to make a match between a bite mark and a set of teeth is false, unreliable and a danger to the discovery of truth and the delivery of justice.
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If there is only one suspect, then we’ll have to make a dental line up. Why dental line up? One is there, we can match it. But that is leading to errors or biases which are commonly three types. One being emotional bias. Dror in his study has said that it is there in our mind that we need to give justice. And this is one suspect so probabilities in our observation itself will change. So that is one, emotional bias. Other is confirmatory bias. Forensic experts are commonly in touch with judiciary and police. So they have idea that these are the probably suspects. So it’s not the one suspect, but suspects are known and our results tend to go in that direction.
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Third bias that can play a role is the observer bias which is called the
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Hawthorne effect where if I am the volunteer, I will bring out the best bite mark on my hand so that the study is successful. So such biases play a role. So we have to make a dental line-up and then compare to the bite mark keeping a blind technique where the doctor does not know and the forensic expert does not know whose sample has been given to him and he or she is matching with the bite mark. Some rigor could be provided in bite mark analysis if the matching process were governed by rules that blinded the matching dentist to the dentition being compared.
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If a series of anonymous dental casts were provided, including that of the suspect, say, ten similar dentitions, and the expert was asked to determine which one of the ten was the closest match, this would be a more robust method. Unfortunately, in my experience, it never happens. One dental cast, one injury, please match. Contextual bias is established and assumptions that the cast must match because why otherwise would this be being asked is established. So after all of the above, what can a forensic dentist reasonably and safely say in an expert opinion on a bite mark injury and a comparison with the dentition?
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That the injury displays the characteristics of a human bite mark. Not always that easy, but often possible. That the injury has gross dimensions that may tell us whether an adult or a young child dentition made the bite mark. Questions of self infliction due to the position of the body may be able to be answered. And perhaps exclusion or inclusion of a suspect as being one possible biter.

Having discussed the various issues with the validity and reliability of bite mark evidence, Dr Bassed and Dr Tarvadi discuss how experts can testify in cases relating to bite marks.

Have you seen reports by forensic odontologists in casework? How have the conclusions been stated? Have you come across judgments in your jurisdiction which rely on bite mark evidence? How do they discuss the reliability of such evidence? Share with us in the comments section.

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Decoding Forensics for Legal Professionals

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