Can a hair really reveal who committed a crime? The FBI has admitted that flawed evidence was accepted in nearly all of the trials that took place over the 1980s and 90s
Since 2012, the FBI has been reviewing 2600 cases in which hair evidence was among that used to secure a conviction. The trials under scrutiny are those that took place between around 1980 and 1999, when hairs were routinely assessed using a microscope 鈥� a technique that has been criticised for its lack of rigour.
Of the 268 trials reviewed so far, more than 95 per cent included evidence that was overstated by analysts. Death sentences were passed on 32 of those cases, and 14 of the individuals have either been executed or died in prison, according to The Washington Post. It is likely that additional evidence supported guilty verdicts, however.
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鈥淚t is not surprising at all,鈥� says Peter De Forest, an independent forensic consultant based in New York. 鈥淚n the 1980s there was a real disconnect between the scientific community and the FBI.鈥� While written reports on hair analysis were often sound and framed with caveats, the evidence tended to be exaggerated when presented in court, he says. 鈥淰ery often the testimony went way beyond the report.鈥�
, a forensic scientist at University College London, also isn鈥檛 surprised. Comparing hairs under a microscope doesn鈥檛 give much away, she says. 鈥淢icroscopic comparison of hairs can be useful in excluding an individual as a possible source of the hair in question, but proposing a potential match between hairs requires DNA analysis.鈥�
Weighing the evidence
Today, DNA analysis of hairs is more commonplace. But even this analysis isn鈥檛 failsafe 鈥� DNA from a cell鈥檚 nucleus can only be isolated if the root of the hair is still attached. Often, analysts resort to mitochondrial DNA, which is passed down from the mother only, and found in the hair shaft itself.
A mitochondrial DNA analysis can only distinguish individuals of different maternal lines, so wouldn鈥檛 allow an analyst to tell the difference between any group of individuals related on their mother鈥檚 side of the family.
Microscopic hair analysis still has an important place in the justice system, says De Forest. He recalls a case where such evidence was used to acquit three men convicted of the rape and murder of a woman.
Initially, the prosecution claimed they found hairs belonging to the victim on the seat of a van used by the suspects. Once the hairs were under the microscope, it was clear to De Forest that they had come from a decomposing body: somehow the hairs collected from the van had mingled with those collected during the victim鈥檚 autopsy.
Unfortunately, this wasn鈥檛 realised until after the suspects DNA analysis of the hair wouldn鈥檛 have helped in this case, says De Forest, because the hairs thought to have been found in the van had indeed belonged to the victim.
鈥淣ow, hair analysis is not done by a number of labs, although it does have value,鈥� says De Forest. 鈥淵ou don鈥檛 want to throw the baby out with the bathwater 鈥� there鈥檚 a danger of missing something.鈥�
