Steven Mark Chaney recently walked out of a Dallas courtroom after his 1987 conviction was overturned because of discredited bite-mark testimony. (Photo: Innocence Project)
Steven Mark Chaney recently walked out of a Dallas courtroom after his 1987 conviction was overturned because of discredited bite-mark testimony. (Photo: Innocence Project)

UA Professor Tackles Faulty Courtroom Science

For years, M. Bonner Denton has been sounding off on the criminal justice system’s science problem. Now a man who spent 28 years in prison on a life sentence for first-degree murder has been released with bite-mark testimony under scrutiny in Texas, where he was convicted.
Nov. 13, 2015

When Steven Mark Chaney was sentenced to life in prison for murder in the first degree, it was bite-mark analysis that did him in. His 1987 criminal investigation was based almost entirely on bite marks, which are now under scrutiny in the state of Texas, where he was convicted.

Last month, 28 years after his conviction, Chaney was released from prison after an amicus brief filed both with the Texas Forensic Science Commission and in Chaney's legal case caused Texas courts, with the consent of the Dallas district attorney, to reverse his conviction. The Texas Court of Criminal Appeals must affirm the decision. 

Meanwhile, the Texas Forensic Science Commission is conducting an investigation into the state's use of bite-mark testimony.

M. Bonner Denton, a professor of analytical chemistry at the University of Arizona, was a signatory on the brief filed on behalf of the Innocence Project.

"I'm concerned with the fact that there is a variety of junk science being practiced as if it were, in fact, valid techniques," Denton said.

"Many are not statistically supported by scientific technology, and bite-mark analysis is, at best, totally junk science. It's highly subjective and dependent on the examiner. It has little if any basis in scientific methodologies."

Given that forensic odontology is still treated as a valid, scientific technique in the American criminal justice system, Denton was compelled to sign the Innocence Project's brief. The project, founded in 1992, is a nonprofit legal organization committed to exonerating wrongly convicted people through the use of DNA testing and to reforming the criminal justice system to prevent future injustice. It is representing Chaney and urged the Texas Forensic Science Commission to conduct its investigation on bite-mark testimony. 

"Your beautician has more required federal government regulation than most forensic laboratories do, and they testify in court," Denton said. And when someone with the title of "forensic scientist" takes the stand, he said, the general public trusts that person's testimony, believing, "'Well, a scientist told me that, so it must be true.'"

According to Denton, the circumstances under which a bite mark makes for trustworthy evidence are very limited.

A fresh bite mark "gets you a good impression, but as it ages, it deteriorates," he said.

The amount of information to be gleaned from a not-fresh bite mark wanes, too. And unless the perpetrator has teeth with a very rare, distinct structure — say, missing every other tooth in the mouth — then trying to identify the perpetrator based on bite marks is almost impossible.

"With a normal individual, you can't determine too much more than the size of their mouth from a bite mark," Denton said.

Denton serves on the National Commission for Forensic Science, a joint effort between the U.S. Department of Justice and the National Institute of Standards of Technology. The commission reviews practices in forensic science with the goal of weeding out inappropriate ones.

Previously, he served on the National Academy of Sciences committee established by the Federal Bureau of Investigation, where he helped identify practices based on appropriate scientific and statistical foundations.

"Do not believe what you see on the popular TV shows," Denton said. "Many areas of forensic science are unfortunately still a long way from being truly scientific."

Denton is now assisting in the appeals of two others who have been convicted of first-degree murder as a result of questionable techniques and testimony from forensic scientists.