Hair and Fiber Evidence: Where Do I Start??

Hair Evidence

Hairs that are collected at crime scenes are often compared to exemplars from known individuals using a microscope.  Based on the microscopic examination, criminalists frequently testify that a particular hair “could” or “could not” have come from a particular individual.  This type of testimony has been admitted in evidence for years but is now recognized as extremely subjective and problematic.  Mistaken conclusions by hair and fiber analysts have been implicated in a number of the false convictions revealed through post-conviction DNA testing.  See, Neufeld, Scheck & Dwyer, Actual Innocence (2002).  These problems undoubtedly arise in part from the total absence of standards for making hair comparisons.  Forensic analysts typically try to classify hairs according to various observable characteristics of color, structure and morphology.  But there is no agreement in the field regarding classification systems and no agreement regarding the number of points of similarity/dissimilarity needed to conclude that a hair could or could not have come from a particular individual.  Moreover, the classifications rest entirely on the subjective judgment of analysts who often fail to use “blind” procedures.  See, Saks, Merlin and Solomon: Lessons from the Law's Formative Encounters with Forensic Identification Science, 49 Hastings L.J. 1069 (1998)    Hence, there is significant danger that analysts will tend to see what they expect to see due to “observer effects” and “examiner bias.”  See, Risinger, Saks, Thompson & Rosenthal,  The Daubert/Kumho Implications of Observer Effects in Forensic Science: Hidden Problems of Expectation and Suggestion, 90 Cal. L.Rev. 1-56 (2002).

In some cases hair analysts have attempted to make statistical estimates of the probability of finding hairs from different individuals that are indistinguishable.  See P.D. Barnett & R.R. Ogle, Probabilities and Human Hair Comparison, 27 J. Forensic Sci. 272 (1982); B.D. Gaudette & E.S. Keeping, An Attempt at Determining Probabilities in Human Scalp Hair Comparison, 19 J. Forensic Sci. 599 (1974)..  However, hair examiners as a group do not generally accept the validity of such statistics.  (See, Peer Review Report: Montana v. Jimmy Ray Bromgard, in the Forensics Library).  Part of the problem is that it is impossible to estimate the frequency of “indistinguishable” hairs when there is no fixed standard for what makes hairs indistinguishable.  Studies that have attempted to determine statistical probabilities have typically used hairs drawn from non-random samples of contributors that may be unrepresentative of relevant population groups.  And these studies have used standards for comparison that may not correspond to standards employed in forensic practice.  Hence, it is widely recognized that these studies do not provide a valid basis for statistical estimation. 

All of this begs the question of whether it is meaningful to tell a jury that hairs are “indistinguishable” in the absence of valid statistics on the probability that such a determination would be reached by chance.  In many states courts have refused to admit evidence of a DNA match in the absence of valid estimates of the random match probability because there is no way for the trier-of-fact to assign meaning to a match without knowing the likelihood the match could arise by chance.  It is clear that a similar problem exists for microscopic hair comparisons, although courts have (so far) not excluded hair evidence on this basis.

The development of Mitochondrial DNA (mtDNA) tests, capable of typing hairs, raises another issue about microscopic hair comparisons.  It is generally believed that mtDNA tests are far more reliable than microscopic comparisons.  Why continue to rely on the microscope when a more reliable method is available?  The answer is that microscopic examinations are quick and cheap.  Some forensic scientists think these examinations are good enough for screening purposes.   Whether they are good enough to constitute reliable evidence is an issue that warrants close examination.