The problem of junk science used as evidence in courts

Because science and its associated technology have been so successful, there is a danger that anything that can be dressed up in the language of science can carry more weight that it merits.

One example is with the use of forensic science in court cases. The ability of modern scientific techniques that can analyze microscopic traces of items at crime scenes and link them to victims and perpetrators (DNA being a good example) has led to the ability to both convict the guilty and exonerate those falsely accused. TV police procedurals also lead to the impression that forensic science is very accurate and even judges can tend to give it greater credibility than it sometimes deserves.

This can result in new techniques being accepted as evidence even when the ‘science’ behind it has not been properly evaluated and is possibly useless, sometimes referred to as ‘junk science’. One example is the so-called science of bite marks.

STEVEN MARK CHANEY had nine alibi witnesses. From morning to night, his movements on June 21, 1987, were well documented. Nonetheless, he was convicted and sentenced to life in prison for the murders of John and Sally Sweek, drug dealers Chaney had previously bought cocaine from and to whom he supposedly owed $500 — his alleged motive for brutally murdering them that day in their Dallas, Texas, apartment. There was nothing to tie him to the crime, save for a supposed bite mark found on John’s arm that several forensic dentists said matched Chaney’s dentition.

Bite-mark evidence rests on a deceptively simple foundation: that human dentition is as unique as DNA; that skin is a suitable substrate to record that alleged uniqueness; and that forensic dentists are adept at identifying wounds, usually bruises or abrasions, that have been made by teeth and then determining whose teeth did the biting. As it turns out, none of these claims are supported by any research, and today bite-mark evidence, which has led to at least 35 wrongful convictions and indictments, is considered junk science.

But while science has discredited the practice, bite-mark analysis is still admissible in many courts. Once a form of evidence is accepted by a judge, legal precedent makes it incredibly difficult to keep even the most questionable forensic practices out of court.

When Chris Fabricant joined the Innocence Project in 2012 to head up its new strategic litigation division, he made it his mission to eradicate bite-mark evidence from criminal cases. In his new book, “Junk Science and the American Criminal Justice System,” Fabricant charts the rise and fall of bite-mark analysis through the group of dentists who created the practice and three of the wrongful convictions that followed, including Chaney’s. He was finally exonerated in 2018.

The article is a cautionary tale about the need to be properly skeptical about things that are claimed to be science, especially when the consequences for the lives of people are so serious.

Another article looks at the rise and fall of bite mark ‘science’.

In his new book, Junk Science and the American Criminal Justice System, Fabricant explains what he means by the term. “Junk science sounds like science,” he writes, “but there is no empirical base for the ‘expert opinion’; it is subjective speculation masquerading as science.”

The world of junk science took off in the late 1960s and 70s – an era in which confidence about the ability of scientists to propel humanity to giddy new heights was unbound. If science could put a man on the moon, then surely it could do the much more mundane job of nailing violent criminals?

Fabricant told the Guardian that there were problems with the new forensic science techniques from their inception. The methods did not arise out of the usual scientific method that starts with a problem, develops an hypothesis to solve it, then tests it via empirical methods.

Rather, it turned the formula on its head. Start with a desired solution – banging up criminals – then work back to the science that would support it.

“Most of the new theories emerged not from a scientific laboratory but from a crime scene,” Fabricant explained. “An enterprising investigator would think, ‘Maybe I could match this suspect’s teeth to the bite on this victim’s nose – that would prove the suspect is the murderer’. Then off they’d go and find an expert witness who could back the theory up.”

There are two stages involved in matching a wound to an individual perpetrator, and both of them are flakey.

The first is categorically to confirm that the wound is caused by someone’s teeth and not by some other sharp object. The second stage is to link the alleged bite mark to the dental pattern of the suspect, by taking a mould of the suspect’s teeth and comparing it to the wound.

Analysis in both those stages, contrary to the scientific method, is subjective. A study published in 2009 by New York scientists tested whether skin could accurately record the impressions left by teeth – the 3D-equivalent of a fingerprint – and found that it could not.

The scientists simulated 23 identical bites on a single piece of un-embalmed cadaver skin. They discovered that the bites produced 23 entirely different marks, each one bearing little resemblance to the rest.

A more detailed study in 2015, carried out by ABFO-certified forensic dentists themselves, asked 38 of their fellow “diplomates” to review photographs presented in real criminal cases where bite mark analysis had been used. Of the 100 cases they examined, the analysts reached unanimous agreement in only four.

These two experiments effectively toppled the entire house of cards upon which bite mark analysis had been erected.

Fabricant is hoping that there will be greater pressure to investigate the convictions of people who have been convicted largely on the basis of junk science. Up to date, 35 people convicted on the basis of bite marks have been exonerated, but some have been executed. Unfortunately it is still admissible as evidence in all 50 states.

You may recall my earlier posts about the case of Melissa Lucio who came within a day of being executed for the death of her two-year old child. Bite marks were used against her in her trial too.


  1. flex says

    Considering that bite-mark evidence comes up with distressing regularity in the late nineteenth-century detective novels I’ve read, it’s not a new phenomenon. A little research suggests that it dates back to the Salem Witch Trials in 1692, but wasn’t formally accepted in the American courtroom until 1954 (Doyle vs. State of Texas. It had to be Texas didn’t it?). That predates Sputnik and the space race, so I’m skeptical that bite-mark analysis was an offshoot of the junk science of the 1960’s and 1970’s. (Note: There was plenty of junk science prior to the 1960’s.)

    I’ve always had a laugh when the late nineteenth and early twentieth century detective novels have a bite-mark as clinching evidence in a crime. The better novelists appear to realize that bite-marks in a wound really can’t show dental features clearly, so they almost always have the murder having left behind an apple with a single bite taken out of it (if they are creative, they use a pear). Apparently murderers, if they eat fruit at all, never eat oranges or bananas, and always only take a single bite, then leave the fruit behind.

    Watson-Jarvis-Hastings, “How do you know the murderer is missing an upper-right canine?”
    Holmes-Thorndyke-Poirot, “Because he was eating an apple and left an impression of his teeth!”
    Watson-Jarvis-Hastings, “By Jove, By Gad, My Word, you’re right! Isn’t is lucky-amazing-surprising, that the murderer took a single bite from the apple just before they murdered that poor woman, and the apple dropped into the pocket of her pinafore without the murderer knowing about it.”
    Watson-Jarvis-Hastings, “It seems a shame that her ward, Master Youngman, happens to be losing his baby teeth, but we best go arrest him.”
    Holmes-Thorndyke-Poirot, “Quite.”
    Master Youngman, “It’s a fair cop.”

  2. sonofrojblake says

    35 people convicted on the basis of bite marks have been exonerated, but some have been executed.

    As if you needed another demonstration that execution is the act of a barbarian “state”.

  3. TGAP Dad says

    For a full, comprehensive look at one particular junk science -- bite marks -- a well as the problem with the entire coroner system, I cannot recommend enough Radley Blake’s excellent The Cadaver King and the Country Dentist. If you think you’re outraged now, just wait. (Note, I don’t post Amazon links, since I just can’t, because Amazon. Consider checking it out from your local public library, instead.)

  4. Mano Singham says

    TGAP Dad,

    Thanks for the this tip. My library has this book and I am going to check it out. The author’s name is Radley Balko, by the way. (Autocorrect is not always our friend!)

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