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Many of the popular crime dramas on television give prominence to forensic evidence. Corpses are identified and murderers caught through analysis of hair, clothing fibres, fingerprints, bite marks, or DNA. A viewer might well get the idea that these techniques are unerringly accurate. But an increasing body of evidence casts doubt on whether forensic evidence is always as clear and accurate as is claimed.

This post cites many reports and analyses raising these concerns. One of the foundational studies questioning the current use of forensics was done 10 years ago by the US National Academy of Science , and the results of that work have very slowly been making their way through the system.

Claims of accuracy are unjustified

Just recently, the Royal Statistical Society in the UK  and the American Statistical Society published a special issue of their magazine, Significance, on forensic science and its limits. In an overview paper Brian Tarran concluded that there is “a worrying lack of science underpinning many types of forensic science evidence”. When experts declare that, say, a hair sample has a very high probability of belonging to a particular person, they make that claim without any “scientifically accepted statistics … about the frequency with which particular characteristics … are distributed in the population”. Tarran says similar doubt exists about fingerprints, the identification of bullets, and bite marks, among others.

Consider this conclusion in a 2019 article in Smithsonian magazine:

Yet it also became clear, over time, that fingerprinting wasn’t as rock solid as boosters would suggest. Police experts would often proclaim in court that “no two people have identical prints”—even though this had never been proven, or even carefully studied. (It’s still not proven.)

An article in ProPublica gives many examples of these misuses and the corresponding wrongful convictions.

Forensic evidence misused

In a 2018 review of research,  Erin Murphy of NYU Law found that cases where people have been exonerated of crimes show consistent misuse of forensics, including DNA, fingerprints, arson, ballistics, hair, and bite marks. Courts often accept as scientific findings that later turn out to be incorrect – as in the case of pathologist Charles Smith in Ontario. In particular, there is too much reliance on and deference to the interpretation and opinion of the expert when claims such as ‘a 1 in a million chance that this was not person x’ are unjustified by scientific data. Courts and defense lawyers do not know enough to be able to challenge this evidence effectively.

Many problems with forensic labs

Another problem in this field has been frequent scandals in labs doing forensic work, in which it is found that the labs are either sloppy or even deliberately misrepresenting their findings. The case of the Motherrisk lab at the Hospital for Sick Children is a recent Canadian example that led to many children being removed from their parents and quite a few parents being wrongfully convicted of serious crimes, with shattering long-term consequences that simply cannot be corrected.

A US article in “American Conservative” reported on some of the many scandals in US forensic labs, including those run by the FBI. That article contains this astounding comment:

… in 2015, the Department of Justice, the FBI, the Innocence Project, and the National Association of Criminal Defense Lawyers (NACDL) released an alarming report that found a 90 percent error rate in 500 FBI cases that the team reviewed. Remarkably, 26 out of 28 FBI hair analysis ‘experts’ provided erroneous statements. Worst of all, several people were executed as a result of this evidence. Of the 35 cases that ended in a death penalty sentencing, 33 included erroneous statements from FBI staff.”

Another FBI report (cited in the ProPublic article mentioned earlier) found that “Hair examiners matched defendants to follicles in 268 trials; all but 11 contained scientific error.”

These are not isolated incidents. Serious problems have been found in the work of crime labs in other places in the US and, as mentioned, in Canada, leading in some cases to thousands of people being cleared of criminal charges but often only years after they were convicted and imprisoned.

An unacceptable situation

One of the major challenges is that lawyers and judges do not have sufficient expertise on these very technical matters to ask the right questions in court. In theory, defence lawyers could do the work to build a challenge to forensic evidence but the cost to clients would be very high, and many people charged with a crime are already struggling to afford a proper defence. Better legal policy is needed to ensure that people are not being convicted on the basis of dubious evidence.



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