Microscopic comparison of hair evidence is a long established forensic discipline. Technically, it’s called “trichology”, and the field dates to the mid-1800’s. Like other visual pattern matching forensics, it had come to be accepted despite the fact that it has essentially no basis in science – just empirical, anecdotal inferences drawn from flawed inductive reasoning. Microscopic comparison of hair evidence has been used as the sole evidentiary basis to not only convict people, but also to execute them. The problem is – it’s not reliable. There are details of two cases cited below in which hair evidence was just plain wrong.
Here’s how it works. This is a diagram of the basic structure of hair:
A hair from the crime scene, or one found on the victim, will be microscopically compared with a number of hair samples from the suspect. The examiner will record observations of characteristics such as the following:
Cuticle Scales _ Flat and smooth _ Protruding or spiky _ Other:
Cortex Thickness _ Thick _ Thin
Cortex Color _ Uniform throughout _ Different colors – Explain:
Medulla Style _ Broken _ Continuous
Medulla Thickness _ Thick _ Thin
Medulla Transparency _ Transparent _ Semi-transparent _ Opaque
The examiner will also take photographs of the crime scene hair and the suspect’s hair side-by-side with a comparison microscope. Then based upon the similarities in characteristics that the examiner sees, he will make a judgement about whether or not there is a “match”. Please note the use of the word “judgement”, as this is just one of the deficiencies with hair analysis evidence – it relies upon the experience, capabilities, training, and judgement of individual examiners.
Now, microscopic hair analysis can reliably tell, based upon visual characteristics, whether a hair is an animal hair or a human hair. It can also tell whether a hair is a head hair, an arm hair, or a pubic hair; but that’s where the reliability stops. It may be possible to suggest that a hair sample is from one of three racial/ethnic groups – European decent, Asian, African – but this is debatable, and is not possible if there is any mixed-race geneaology involved.
Two cases in which hair comparison evidence was wrong:
1) Please see Mark Godsey’s post from April 28, 2012 citing the Santae Tribble case in Washington D.C. Tribbles murder conviction was overturned when DNA testing proved that the hair evidence that was used to place him at the murder scene and to convict him was not from him.
2) Then there is the tragic case of Claude Jones in Texas. Jones was executed for murder in December, 2000, after a request for a 30-day stay to perform DNA testing on the hair evidence that convicted him was denied by then governor George W. Bush. Ten years later, the Innocence Project was able to get a Texas judge to order DNA testing. The DNA testing proved that the hair evidence used to convict and execute Jones did not match him. Here is a link to details of the case:
NAS Report: Here is how the NAS Report (https://wrongfulconvictionsblog.org/2012/02/20/the-sorry-state-of-forensics-in-the-us-and-perhaps-the-world-2/) sums up microscopic hair analysis evidence:
1) No scientifically accepted statistics exist about the frequency with which particular characteristics of hair are distributed in the population.
2) There are no uniform standards on the number of features on which hairs must agree for an examiner to declare a “match”.
3) The categorization of hair features is largely subjective, and depends heavily on examiner proficiency and practical experience.
4) The report finds no scientific support for the use of hair comparisons for individualization in the absence of nuclear DNA.
[Editor’s Note: Since there are no scientifically accepted statistics for the frequency of occurrence of various hair characteristics within the general population, there can be no knowledge of whether or not various characteristics occur with statistical independence. That is, the occurrence of one characteristic having no statistical relation to the occurrence of any other characteristic. So …. even if we did know the frequencies for various characteristics, we could not multiply them to compute the overall frequency of occurrence for a combination of characteristics. Hair comparison analysis is yet another forensic discipline that fails the test of “Show me the data from which I can compute a probability of occurrence.”]
The Innocence Project of New Orleans has drafted a template Daubert motion to exclude microscopic comparison of hair evidence. It is Louisiana-specific, but is easily adaptable to other jurisdictions. I include it here for your reference:
Wrongful Convictions Blog 
Pingback: Why I Think the US Justice System is Broken – and Why It’s Not Getting Fixed | Wrongful Convictions Blog
Pingback: Hair Analysis Evidence About to Join CBLA as “Junk Science” | Wrongful Convictions Blog