If you've been reading our blog, you know that many common forensic evidence techniques have been called into serious question by scientists. Yet police, prosecutors and judges continue to use or allow these techniques to be used. Sometimes, it seems as if they are used simply because they have been in use for so long. That isn't sound policy.
"If we don't have technologies that are objective, repeatable and reliable, then we have no idea how many times we're making the wrong decision," says the director of the Center for Statistics and Applications in Forensic Evidence, a government-funded project aimed at determining the useful limits of forensic evidence. "We don't even have a way to estimate how many times we're making the wrong decisions."
Recently, NBC News published an in-depth report on the problems with common forensic techniques. It's far too detailed to cover in a blog post, and we recommend you read the entire article.
According to the National Registry of Exonerations, since 1989 at least 553 exonerees were originally convicted based on false or misleading forensic evidence. These exonerations probably represent only a tiny portion of all wrongful convictions based on bad forensic science, according to researchers.
As research has shown before, several forensic techniques commonly used in criminal prosecutions are scientifically questionable, at best. Examples include:
- Bite mark comparisons
- Footwear analysis
- Blood-spatter analysis
- Hair and fiber analysis
- Tire tread analysis
When it comes to bite mark comparisons, for example, academic studies and government reports have found insufficient evidence to prove that bite marks are unique or that skin imprints record bite marks accurately. In fact, it has yet to be shown that forensic dentists can actually identify bite marks reliably.
The research has been done by impressive groups, such as the President's Council of Advisors on Science and Technology, the National Academy of Sciences, and the Texas Forensic Science Commission. Yet judges may be reluctant to rule that these questionable techniques aren't generally accepted by the relevant scientific community, the standard for admissibility of expert testimony.
The trouble is, once these techniques are admitted as evidence, they can seem much more convincing than they truly are. This is the reason we require testimony based on forensic evidence to be generally accepted by the scientific community; so untried "science" won't unduly influence the jury.
For the last decade, scientists have repeatedly warned that these techniques are much less certain than forensic examiners testify they are. Forensic examiners have pushed back, despite multiple exonerations and repeated scandals, including the FBI walking back on numerous convictions it obtained using hair analysis.
This makes it seem as if there's a scientific dispute about the techniques. There is not.