Bite-marks giving false impression
ON 8 April 2002 Ray Krone walked out of prison in Yuma, Arizona, having spent 10 years behind bars, including two on death row.
His conviction, for stabbing cocktail waitress Kim Ancona to death, was secured largely on the basis of a supposed match between his teeth and a bite mark on the victim's breast.
Krone always maintained his innocence, and was eventually exonerated when DNA on the victim's clothes was matched to another man.
The prosecution expert - who was certified by the American Board of Forensic Odontology (ABFO), the professional society for experts in forensic bite marks - told the jury which convicted Krone that for bite marks "a match is 100 per cent".
But critics of such evidence argue that the technique is always subjective and has never been held up to rigorous experimental validation.
Christopher Plourd, the San Diego attorney who campaigned for Krone's release, told New Scientist, "This is not a science." He claims miscarriages of justice like Krone's case are all too common.
Now debate is being further stirred up by a study from two California dentists with experience in forensic cases, which they say validates the technique.
George Gould of Rancho Murieta and Anthony Cardoza of El Cajon admit their research is preliminary, but they claim it shows bite-mark matching is accurate under certain ideal conditions. "The technique is reliable with a high degree of accuracy," says Gould. But critics are still far from convinced. The study says little if anything about real-life scenarios, they argue, because the skin marks the researchers used were much clearer than those in real-life cases.
"Bite marks don't lend themselves well to a bench study," says Richard Souviron, a forensic odontologist at the Miami-Dade Medical Examiner's office. Even with these unusually clear bite marks, some subjects in the study were falsely identified while others were falsely excluded.
Gould and Cardoza presented their research to the American Academy of Forensic Sciences conference in Dallas, Texas, last month. They made marks using casts of 10 different sets of teeth in either clay or human skin (Gould's arm).
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 are hours or days old. The indistinct bruises and abrasions that result are often difficult to identify as bite marks, let alone match to a killer.
In a case discussed at the AAFS meeting, bruises made by a hairband had been matched to the suspect's dentition. There is also the compounding factor that forensic odontologists generally know which set of teeth belongs to the main suspect.
This might make them more likely to come up with a positive match for that person. Cardoza admits that bite-mark evidence will always have a subjective element: "The best bite mark is one you can swab for DNA." He says the technique works best when there is a limited number of possible suspects: for example, if other evidence indicates only two or three people could possibly have committed the crime.
If cases like Krone's are to be avoided in future, Plourd says, the ABFO must urgently review the technique, and raise the standards used to analyse bite mark evidence. "They owe him something."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
Published on PsychCentral.com. All rights reserved.