Fingerprint Individuality

Abstract

Expert testimony based on forensic evidence (such as handwriting, fingerprint, hair, bite marks, etc.) is routinely collected and presented in courtrooms. Among the various sources of evidence, fingerprints have been used in courts of law for almost 100 years and the testimony based on fingerprints carries substantial credibility and weight. The use of fingerprint evidence involves comparing salient features of a latent print lifted from a crime scene with those of rolled (full) impressions taken either from the known defendant or the entire criminal database. A reasonably high degree of match between the salient features leads the latent experts to testify irrefutably that the owner of the latent print and the defendant are one and the same person. For decades, the testimonies provided by latent print experts were almost never excluded from these cases, and on cross-examination, the foundations and basis of such testimonies were rarely questioned (Cole (2001a, b)). Central to establishing an identity based on fingerprint evidence is the assumption of discernible uniqueness; salient features of different individuals are observably different, and therefore, when two prints share many common features, the experts conclude that the owner of the two different prints is one and the same person. The assumption of discernible uniqueness (Saks and Koehler, 2005), although lacking sound theoretical and empirical foundations, allows forensic experts to offer unquestionable proof towards the defendant's guilt, and to make matters worse, these experts are never questioned on the uncertainty associated with their testimonials (that is, how frequently would an observable match between a pair of prints lead to errors in the identification of individuals; see Haber and Haber (2004)). Thus, discernible uniqueness precludes the opportunity to establish error rates which would be known from collecting population samples, analyzing the inherent feature variability, and reporting the corresponding probability of two different persons sharing a set of common fingerprint features. A significant break from this trend occurred in the 1993 case of Daubert v. Merrell Dow Pharmaceuticals (113 S. Ct. 2786) where the U.S. Supreme Court ruled that in order for expert forensic testimony to be allowed in a court case, it had to be subject to three main criteria of scientific validation, that is, whether the particular tool or methodology in question (i) has been tested, (ii) has been subjected to peer-review, and (iii) possesses known error rates. Following Daubert, fingerprint identification was first challenged in the 1999 case of U.S. v. Byron Mitchell (Criminal Action No. 96-407, US District Court for the Eastern District of Pennsylvania) under the premise that the uniqueness of fingerprints has not been objectively tested and matching error rates are unknown (also see Newman (2001)). Based on the outcome of U.S. v. Byron Mitchell, fingerprint based identification has been challenged in more than 20 court cases in the United States (for example, U.S. v. Llera Plaza 179 F Supp 2d 492 ED Pa 2002 and 188 F Supp 2d 549 Ed Pa 2002 and U.S. v. Crisp 324 F 3d 261 4th Cir 2003). Cole (2006) has compiled a list of 22 known exposed cases of erroneous fingerprint identifications made by forensic experts.