This is the first post in a series I will be doing called “Don’t Trust the Experts” – a look at certain forensic techniques that are admissible in Australian courts, but have questionable validity. Too often lawyers ‘trust the experts’ when it comes to forensic methods and conclusions, even though such methods have very little empirical support. First on the chopping (or should I say ‘chomping’) block: “Bite Mark Analysis”.
Forensic odontology or ‘forensic dentistry’ is one of the oldest forensic techniques around. However, just because a field is well established doesn’t mean that it is scientifically valid. This is particularly true of a subsection of forensic dentistry known as ‘bite mark analysis’.
Bite mark analysis generally involves the identification of a suspect by comparing impressions taken from bite marks on a victim, perpetrator, or on foodstuffs found at a scene with dental records or impressions of real-life subjects.
Despite widespread use and reliance on bite mark analysis to identify suspects, there is very little empirical support for the method’s underlying presumptions or its efficacy. Indeed bite mark evidence was strongly condemned in the 2009 National Academy of Science report on ‘Strengthening Forensic Techniques in the United States‘, which concluded:
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.
Compared to US standards, Australia has been far more liberal in allowing bite mark evidence to be admissible to prove identification. This is particularly worrying, as wrongful convictions as a result of bite mark evidence are widely documented. Lawyers, judges and experts need to be more aware of the failures of this method of identification.
The deficiencies of bite mark analysis as a forensic technique centre around certain unavoidable factors:
- Firstly, skin is an unreliable medium to leave a dental impression, making comparison between an impression and a real-life suspect very difficult.
- Secondly, human teeth are subject to considerable change over time through chipping, erosion, teeth loss and other changes making ‘perfect’ matches between suspects and bites very unlikely.
- Thirdly, there is little consistency in the method of comparison used by different forensic dentists in comparing a bite impression to a suspect. Nor are there enough studies to identify a ‘best practice’ method.
- Fourthly, identification relies on the presumption that dental impressions left by a bite are unique enough to identify a suspect. However, most bite marks are usually partial impressions which are unlikely to demonstrate statistically significant uniqueness.
- Finally, many convictions which were made on the basis of bite mark analysis have since been overturned due to new DNA evidence, indicating that it is an unreliable method of identification.
Despite the failures noted above, this sub-set of forensic odontology still has some role to play in the legal system. Having a dentist look at bite marks may still be useful in determining whether a particular bite is of human or animal origin or whether a particular injury was caused by teeth. One decade long survey of bite mark cases in Australia concluded that the majority of bite mark evidence led in court, were not used for identification purposes.
Nevertheless, lawyers, judges and experts need to be more vigilant in how they use this technique in the future. In particular, we need greater legal challenges to this expert evidence being admissible in courtrooms to prove positive identification.