History
The use of microscopic hair comparison in criminal cases can be traced back to the mid-19th century, with one of the earliest examinations occurring in 1847 during the murder investigation of Duchesse de Praslin in France. The theoretical foundation for microscopic hair analysis was established by French criminologist Edmond Locard, who proposed that “every contact leaves a trace,” suggesting that people continuously transfer particles of hair and other evidence without knowing. In the United States, microscopic hair analysis was first considered in 1882 in Knoll v. State, where an expert testified that hair found in a suspect’s wheelbarrow matched the victim’s hair in “length, magnitude, color, and in every other respect.” However, this case was reversed on appeal, with the court warning that such evidence was “of a most dangerous character.”
Despite these early concerns over its validity, microscopic hair comparison analysis developed into a common forensic technique. The process generally involved examining the morphology of hair strands, including the scale type of the cuticle, pattern of the medulla, and pigmentation in the cortex. Analysts used pattern recognition to compare unknown hairs to sample hairs, typically reaching the conclusion that the hairs were “consistent with” each other or “microscopically indistinguishable.” The FBI, which first started using microscopic hair comparison in the 1950s, maintained that properly trained technicians following the correct procedures should reach the same conclusions.
Problems with microscopic hair analysis began to emerge as DNA testing became available. Despite the advancement in DNA testing, hair comparison continued to be used when DNA testing was deemed too time-consuming, expensive, or otherwise unavailable. In 1994, the U.S. Department of Justice established a task force to review cases handled by an overzealous government witness. By 1996, the DOJ reported that 28 people, many convicted based on microscopic hair comparison, had been exonerated through DNA testing. A 2002 study comparing microscopic hair comparison with mitochondrial DNA sequencing conclusively demonstrated that DNA typing was far more accurate.
The unreliability of microscopic hair comparison had serious consequences. Roger Coleman was executed in 1992, and hair comparison played a significant role in his conviction, despite lingering questions about his innocence. While results of hair comparison are subjective, the prosecutor in Mr. Coleman’s case told his jury that based on scientific analysis of the hair, “It would be extremely unlikely that anyone else would have hair that would be consistent with this hair.” In an interview, the trial judge later acknowledged that the expert witness at the trial never compared the hair found on the victim in Mr. Coleman’s case to anyone other than Mr. Coleman.
In April 2015, the FBI announced that a review of their microscopic hair comparison analysis testimony in 268 cases revealed FBI expert witnesses gave erroneous testimony in 96% of cases. Eventually the FBI flagged nearly 3,000 files where analysts may have provided improper hair comparison evidence. A 2018 update from the FBI revealed that erroneous testimony was found in 93% of 484 reviewed cases. Nine people whose convictions partially relied on improper hair comparison had already been executed, and five more had died in prison.
Standard of Care
Macroscopic characteristics of hair, those that can be observed without a microscope, include length, color, and texture. However, these characteristics alone are not enough to determine a hair’s origin. The first steps in the hair comparison process involve determining whether the unknown hair sample is of human or animal origin. Microscopic characteristics of hair include the medulla, pigmentation of the cortex, and the types of scales present in the cuticle. These characteristics differ in human and animal hair.
Labeled diagram of a hair follicle.
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
The medulla, or innermost layer of the hair shaft, if present, is made up of pigment. The medulla may be a continuous, unbroken line of pigmentation, or may be interrupted, fragmented, solid, or not exist at all. An interrupted medulla includes pigmentation broken up in regular intervals, while a fragmented medulla has pigmentation broken up in uneven increments. A hair strand with a solid medulla means that pigmentation is found throughout the entire hair, both in the medulla and the cortex (largest part of a hair shaft, layer surrounding the medulla). Generally, the cortex of human hair contains pigment granules, or melanin, that give the hair color. Pigment distribution varies from individual to individual, as well as the size of the pigment granules found in the cortex.
While pigmentation in human hair tends to be denser towards the outside of the cortex, pigmentation in animal hair tends to be denser towards the medulla. Human hairs have generally consistent color and pigmentation throughout a strand, whereas animal hair shows varying color changes throughout a strand. Another prominent difference appears in the medulla: In humans, the medulla is thin and takes up no more than 1/3 of the hair’s total width. In animals, the medulla tends to be much wider (half or more than half the hair’s total width), and more well-defined. The cuticle, or outermost layer of a strand of hair, is made up of overlapping scales that protect the inner layers of the hair. Different mammals have different hair cuticle patterns: rodents have coronal cuticles that look like a stack of crowns, while cats have spinous scales that resemble petals. The scales of human hair are flat and narrow, called imbricate.
Human hair under a microscope
Juan de Vojníkov, CC BY-SA 4.0 <https://creativecommons.org/licenses/by— sa/4.0>, via Wikimedia Commons
For animal hairs, examiners can determine the specific species, though they cannot definitively determine the single animal the hair came from. Through analysis, animal hairs recovered from evidence might establish connections, potentially tying an individual to a crime scene, vehicle, or location of a victim. When examining human hairs, analysts must look at the arrangement of scales, distinctive appearance, and distribution of hair characteristics across different hair regions.
When comparing a known hair to an unknown hair, an examiner must use a comparison microscope, which is made up of two compound light microscopes connected by an optical bridge. This configuration allows an examiner to look at a known and unknown hair simultaneously. There are typically two conclusions an examiner can reach: (1) the hair samples are “not consistent with” each other or (2) “microscopically indistinguishable.” This microscopic comparison can only determine whether the unknown hair sample exhibits similar characteristics to the known sample and whether it could have originated from the same source.
If these hairs are deemed to be consistent with one another based on microscopic findings, examiners may use DNA and blood protein testing to determine if they came from the same individual. Microscopic assessment is performed first because it is more cost effective and faster than DNA and blood protein testing. Individual strands of hair can only be attributed to an individual if the follicle is attached to it. If a hair is forcibly removed, the follicle may stay intact, allowing analysts to examine the follicular tag, where blood and tissue from the individual may be present. DNA testing of this material provides identification to a high level of confidence, whereas simple hair comparison cannot provide any definitive identification of an individual.
Concerns with Microscopic Hair Comparison
Microscopic hair comparison analysis suffers from significant scientific limitations, as the analysis relies on subjective assessment from examiners rather than objective, quantifiable measurements, allowing for inconsistency between analysts. This technique can only indicate whether a questioned hair is “consistent with” a known sample, creating a potential class of contributors, rather than identifying an individual source.
Forensic analysts have historically overstated the significance of their determinations in testimony, implying levels of certainty that science simply does not support. As found in the National Registry of Exonerations’ (NRE) December 2023 report, Microscopic Hair Comparison Analysis and Convicting the Innocent and the FBI’s review of microscopic hair comparison analysis cases, FBI “experts routinely used erroneous statements to make the evidence sound stronger than it had sounded in written reports.” Testimony from these experts repeatedly exceeded the limits of the science in question. NRE’s review of exonerations involving microscopic hair comparison analysis determined that at least 129 individuals had been falsely convicted based at least in part on this evidence, with 15 of these exonerees having been sentenced to death.
“Even when the testimony was carefully phrased, it contributed to the conviction of innocent people…I suspect that this is because the appearance on the stand of an expert witness, who said they were using science and the evidence implicated the defendant, led the jury to think that the evidence must have been powerful evidence of guilt, even if the expert avoided saying it explicitly.”
Case Studies
Gary Nelson
Gary Nelson spent 11 years on Georgia’s death row after being convicted of the 1978 rape and murder of six-year-old Valerie Armstrong.
The case began in February 1978, when Valerie and her eight-year-old friend S.W. had been playing together, and they decided to walk to the store. When Valerie didn’t have money for candy, she told S.W. she would ask “Uncle Al” or Alphonso Swinton, for money. At this time, Mr. Swinton shared a house with Gary Nelson. According to S.W., when they arrived at Uncle Al’s house, they parted, and the last time she saw Valerie, she was walking toward the door while a man worked on a car in the driveway. Valerie’s body was discovered the next day in the woods behind her house, with a broken knife found nearby.
Mr. Nelson, who had prior minor run-ins with the police and lived in the area, became an immediate suspect. Although he voluntarily met with police a week after Valerie’s body was found and allowed his house to be searched, he was arrested in May 1978. Mr. Nelson was indicted by a grand jury in October 1979. The state had no direct evidence linking Mr. Nelson to the crime — no eyewitnesses, fingerprints, or confession — only circumstantial evidence based on hair analysis.
The hair evidence was central to the prosecution’s case against Mr. Nelson. Roger Parian, director of the Savannah Branch of the State Crime Laboratory, testified that he had mounted a hair found on Valerie’s body on a slide, examined it, and determined it came from the arm of a Black person. On the stand, he claimed he could narrow down the source of the hair to about 120 Black people out of Chatham County’s 60,000 Black residents, with Mr. Nelson included among those 120 individuals. District Attorney Andrew J. Ryan III emphasized this evidence in his opening statement to the jury, telling them that “the hair that was found on the body and the known hair from the arm of Gary Nelson have, in (Mr. Parian’s) opinion, the same origin.”
Based on this and other evidence, the jury convicted Gary Nelson and sentenced him to death. Shortly thereafter, an Atlanta law firm agreed to represent Mr. Nelson on appeal pro bono. After ten years of investigation, the defense team uncovered crucial evidence that prosecutors deliberately withheld despite proper motions and a direct order from the trial judge to comply with discovery.
The hair evidence, which had been pivotal to the state’s case, fell apart. Contrary to Mr. Parian’s sworn testimony, he had never actually examined the hair found on Valerie Armstrong’s body. Rather, at his request, the FBI’s crime lab in Washington, D.C., examined the hair and sent its report to Mr. Parian. The FBI concluded that “this hair is not suitable for significant comparison purposes.” This report had been withheld from Mr. Nelson’s counsel. Myron Scholberg, the former Unit Chief of the Microscopic Analysis Unit of the FBI’s crime lab, provided an affidavit stating that “the limb hair obtained from the victim could have come from any Black person, including, but not limited to, other suspects in this case of the victim.” This affidavit exposed Mr. Parian’s false testimony and the supposed forensic evidence linking Mr. Nelson to the crime scene.
The prosecution’s files contained substantial evidence that was deliberately withheld from the defense: statements from multiple witnesses corroborating Mr. Nelson’s alibi that he was elsewhere when Valerie disappeared; concrete evidence pointing to someone else as the perpetrator, including a confession; and information that multiple witnesses saw Valerie after she left the house where Mr. Nelson was allegedly working on his car.
Despite the acknowledgment of the withheld evidence, in May 1990, Judge E. Byron Smith upheld Mr. Nelson’s conviction, claiming the withheld evidence would not have made a difference in the trial outcome. In November 1991, the Georgia Supreme Court unanimously disagreed with Judge Smith and held that Mr. Nelson’s conviction and death sentence were obtained illegally. Then-Chatham County District Attorney Spencer Lawton, who had not prosecuted the case, acknowledged, “There is no material element of the state’s case in the original trial which has not subsequently been determined to be impeached or contradicted.” The district attorney chose not to retry Mr. Nelson, and he was released from prison on November 6, 1991.
Sources
Rene Ebersole, How the Junk Science of Hair Analysis Keeps People Behind Bars, Mother Jones, December 15, 2023; David S.H. Funes, Kaitlyn Bonilla, Mathieu Baudelet, Candice Bridge, Morphological and chemical profiling for forensic hair examination: A review of quantitative methods, Forensic Science International, 2023; Samuel D. Hodge and Amelia Holjencin, A Post-Mortem Review of Forensic Hair Analysis – A Technique Whose Current Use in Criminal Investigations is Hanging on by a Hair, Saint Louis University Law Journal, April 7, 2020; Douglas W. Deedrick, Hair, Fibers, Crime, and Evidence, Part 1: Hair, FBI Laboratory Division, July 2000.
See also Killing Justice: Government Misconduct and the Death Penalty