Hair as forensic evidence
From Wigipedia, the free hair encyclopedia
A single hair left at a crime scene feels like powerful evidence, and for decades it was treated that way. Analysts would place a crime-scene hair and a suspect’s hair side by side under a comparison microscope and describe how their color, thickness, and internal features lined up — then tell the jury the two were consistent, or even “matched.”[citation needed]
What microscopy can and cannot do
Microscopic examination genuinely can do useful things. It can tell a human hair from an animal’s, a head hair from a body hair, a dyed hair from a natural one, and it can exclude a suspect whose hair looks nothing like the sample.
What it cannot do is identify a specific person. There is no established way to say that two visually similar hairs came from the same head and only that head. The judgment rests on an examiner’s eye, and “looks the same” is not the same as “is the same.” For years, that gap was papered over with confident courtroom language about probabilities that the science could not actually support.
The 2015 reckoning
After several people were exonerated whose convictions had leaned on flawed hair testimony, the FBI and the Department of Justice reviewed the work of the bureau’s own hair-comparison unit. The findings were stark: examiner testimony contained erroneous statements in at least 90 percent of the trial transcripts reviewed, and 26 of 28 examiners had overstated matches in ways that favored the prosecution. Some of the affected cases were capital cases.
The review focused on work done before 2000, the point at which mitochondrial DNA testing on hair became routine.
Hair evidence today
Hair has not left the courtroom — it has been demoted to honest work. A hair shaft can yield mitochondrial DNA, which narrows identity far more rigorously than a microscope ever could, and hair can still reveal drugs, toxins, and animal-versus-human origin. The lesson that stuck is a plain one: similar is not the same, and a confident expert is not a measurement.