They photographed the marks and gave them to 22 experts along with "overlays" of the casts - images of the pattern a set of teeth make when they bite onto a flat surface.
Forensic odontologists generally place such images over a wound to decide whether the two match. The experts in the study were asked to match the overlays with images of the clay or skin marks, using a sliding scale of certainty.
Gould and Cardoza told the conference that the experts correctly matched 98 per cent of the clay marks and 84 per cent of those on skin. On the face of it, this is a good result, but the pair admit these figures exaggerate the success rate, because in some cases they included some only labelled as "possible" matches.
What's more, in some cases the experts excluded the correct cast, saying they were certain it could not have made the mark. And there were examples where they assigned the wrong cast to a mark, a false match which in a real case could have led to a miscarriage of justice.
Gould and Cardoza wouldn't tell New Scientist how many of these errors there were. It seems likely that such errors would be much more frequent in real cases.
In the study, the experts were given ready-made overlays, so they did not have to produce their own, removing one source of variation. More significantly, the skin marks were photographed immediately. "I don't think anyone is going to see a photo taken immediately after the bite unless it was submitted by the suspect," Gould admits.
None of the marks left skin broken, red or bruised and the experimental "victim" did not struggle. "Evidence is never of that good quality," says Plourd.
In real cases, forensic odontologists examine marks that a
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Contact: Claire Bowles
claire.bowles@rbi.co.uk
44-207-331-2751
New Scientist
10-Mar-2004