Forensic bite-mark matching has come in for severe criticism. A handful of recent murder convictions in the US that relied on bite-mark evidence were overturned after DNA evidence came to light. And previous research has shown that even under ideal conditions matching bite marks is fallible (New Scientist, 13 March 2004, p 6)
One major problem is the way the evidence is collected. Odontologists usually have to try to match photographs of bite marks to "overlays" of the suspect's teeth- 2D images of the pattern that teeth make when they bite onto a flat surface. But bite marks that curve around arms and fingers look different. "More than half of the photos we receive get sent back because they are too distorted," says David Sweet, director of the Bureau of Legal Dentistry at the University of British Columbia in Vancouver, Canada. "It's not a reflection on the police photographer, it's a reflection of the difficulty of the job." But, says Sweet, bite marks are present in 8 out of 10 sexual assault and homicide cases and can be valuable evidence.
The solution might be to use 3D imaging to match teeth to a bite. That's how John Clement and Sherie Blackwell at the University of Melbourne, Australia, set out to overcome the distortion problems.
Blackwell asked 42 dental students to make a mould of their own teeth. The students then chomped onto wax blocks to mimic a bite mark on skin. Both the teeth moulds and sample bites were digitised with 3D laser scanning. Blackwell then used a statistical technique to estimate the probability of a match. Worryingly, the system proved to have a 15 per cent chance of matching a bite to the wrong person. Only four bites matched their corresponding set of teeth and no other. Twenty-seven bites matched their owners' teeth and at least one other person's, while seven matched the wrong teeth altogether, and four matched none of the teeth.
However, the system should do better in real cases, says Blackwell, who presented the results at the American Academy of Forensic Sciences meeting in New Orleans, Louisiana, last month. The students' teeth were all straight and unbroken, giving few of the distinguishing characteristics that forensic odontologists look for. "Often the people involved in criminal activity aren't that fortunate," says Sweet. "They produce bite marks that are easier to identify than 'piano-key' teeth."
But even without definite matches, the technology could be useful in courtrooms. Clement suggests that experts could show the animations to jurors, letting them see with their own eyes the likelihood of a bite-mark match. At present, jurors have to rely on expert testimony to assess the reliability of bite-mark evidence.
Sweet agrees. "Bite-mark evidence is complex and difficult to explain. Anything you can do to illustrate the scenario is advantageous," he says.
Anna Gosline, New Orleans
This article appears in New Scientist issue: 12 MARCH 2005
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