There were bite marks on John Sweek's arm when police found him stabbed to death on his kitchen floor in 1987.
Dallas County prosecutors charged Steven Chaney with the murder — he was a construction worker who had bought drugs from Sweek. And those bite marks were going to hurt. During his trial, the linchpin in Chaney's conviction came from a forensic dentist, the chief dental consultant for the Dallas County Medical Examiner's Office, who testified that there was a “one to a million chance” that someone other than Chaney had bit Sweek's arm. Hard to argue with that.
But nearly 30 years later, a bit of a different perspective has emerged. “I think pretty much everybody agrees that there is no scientific basis for a statistical probability associated with a bite mark,” said Dr. Henry Kessler, chair of the Texas Forensic Science Commission’s subcommittee on bite mark analysis. “That's just one of the things that have led people to recant their [bite mark analysis] testimony — particularly in the case of Steven Chaney.”
Chaney was freed from prison last fall after serving 28 years. His release came after experts and prosecutors agreed that the bite mark analysis used to secure his conviction was nothing but junk science. At that time, the Forensic Science Commission had already been studying Chaney's case for months — and it would ultimately lead the commission to release a recommendation last week that bite mark analysis evidence should not be admitted in court.
Last fall, members of the commission asked forensic dentists who staunchly support bite mark analysis to provide studies or research that would validate the science in the face of so much doubt (and not just in Chaney's case — he is the 26th person to be wrongly convicted because of such "evidence"). The literature that threw shade on bite mark analysis had appeared to trump any of the science's redeeming qualities, and until the commission could see some more research, it wasn't going to support it, Kessler explained.
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The first problem with bite mark evidence, Kessler told the Houston Press, is that often experts can't even agree with each other on identifying human bite marks in the first place. “That was something that was kind of disturbing,” Kessler said. So the commission charged the bite mark supporters to come up with criteria that would allow experts to reliably recognize human bite marks — guidelines of sorts.
Another problem is the malleable qualities of our skin. “It's elastic, it rebounds, blood flows through it, causing the marks to change over time,” Kessler said. “And so the quality of the evidence is often very poor.” The commission charged the dentists with creating a quality threshold that the evidence would need to meet.
Which means, of course, that there may be a way that bite mark evidence can be used in the future. Not as linchpin evidence, but as supplemental evidence used to bolster a case. An example, Kessler said, is if experts swab a bite mark for saliva DNA analysis, then find that the suspect also has chompers that match the bite mark. “Everyone involved in this would agree that if the only evidence you have is a bite mark, you're on really shaky ground,” he said.
The FSC is currently reviewing several other cases of defendants whose convictions relied heavily on bite mark analysis. They include the case of David Spence, who was executed in 1997 after he was convicted of killing three teenagers in Waco on nothing but bite mark evidence. One of the forensic dentists who testified in his trial was the same one who helped convict Steven Chaney.