By Andre A. Moenssens1
Fingerprint identification has been around for a long time. It has nearly a century of court acceptance in the United States. Yet, in the aftermath of United States Supreme Court cases like Daubert v. Merrell Dow Pharmaceuticals (1993) and Kumho Tire v. Carmichael (1999), requiring courts to determine the reliability (validity) of underlying techniques before admitting expert opinion based on it, questions are bound to be raised about the scientific legitimacy of many of the techniques commonly used in crime laboratories, fingerprint identification among them.
Skilled examiners of fingerprint evidence agree that the process of comparing latent fingerprints of unknown origin with inked impressions of known origin is an "art," rather than a science. It requires an examiner to assess, on the basis of experience in dealing with thousands of fingerprints, what parts of an incomplete and partially blurred latent print show visible friction ridge detail that can be used for identification purposes. But whether fingerprint identification is "art" or "science" is clearly no longer relevant to a Daubert inquiry. What needs to be examined is whether the underlying premises upon which fingerprint identification are based have been empirically validated. And these premises are three-fold: (1) the friction ridge detail of the epidermis on the palm side of the hands2 remains unchanged during the lifetime of an individual, except for accidental or intentional scarification or alteration; (2) friction ridge pattern areas exhibit so much variety of detail that no two patterns are ever found to be exactly the same on the digits (or palms or soles of feet) of one individual or on the digits (or palms or soles of feet) of other individuals; (3) while these friction ridge patterns exhibit an infinite variety of detail, they nevertheless fall within certain broad classes or categories that permit police to store and retrieve millions of prints according to classification formulae.
Premise one has clearly stood the test of time and experience. It has been established in over 100 years of accumulated experience that friction ridge patterns remain unchanged naturally in their ridge detail during the lifetime of an individual. The ridge patterns begin to form during pre-natal life and are fully formed by the seventh month of fetal life.
Premise three has equally proved to be true by verification and experience. We have been able to deal with millions of accumulated fingerprint cards by devising classification formulae based on pattern types and subgroups3 until the advent of automated computer and retrieval systems, referred to generically as AFIS4 systems in "the trade," made such classification formulae unnecessary.
Premise number two, that all fingerprints are unique and never duplicated in the universe, is a premise that is harder to prove empirically, despite the fact that all fingerprint examiners fervently believe in it. However, in all of the human experience with fingerprints world-wide, no two fingerprints from different digits have ever been found to match exactly. It has been argued that, since millions of sets of prints have been stored in fingerprint files as voluminous as, say, the FBI Identification Section and no exact duplication of friction skin detail has been encountered in these fingerprint repositories, individuality is clearly proved. The problem with this assertion is that it does not stand the test of reason. The millions of sets of prints were never compared against one another for possible duplication of friction ridge patterns. Filing and retrieving prints from such a massive file only results in an examination of a comparatively small number of sets of prints: those with a matching, or approximately matching, classification formula.
There is, however, respectable empirically established evidence of the uniqueness of fingerprint patterns. Studies done by many examiners have shown that the fingerprints of identical twins are different, as are the prints of triplets, quadruplets, and quintuplets. In that sense, fingerprint identification has been found to be even more discriminating than the vaunted DNA (deoxyribonucleic acid) "fingerprinting" method, which cannot distinguish, by today’s technology, between the DNA of identical twins. Since inherited traits for similarity in patterns and sub-pattern types are the most common among people who are very closely related, the difference in the prints of such persons certainly can be taken as empirical evidence of fingerprint individuality. Might we not infer from that experience that all fingerprints of different digits are, indeed, different?
Persons skilled in fingerprint identification, who have literally viewed, scanned, and studied tens--if not hundreds--of thousands of individual patterns, do not doubt this. Clearly, if exact pattern duplication were to exist in the world, at least a single instance of this would have been discovered by now. While such claims have been made often, every case, when examined, has established that the prints of different digits that were allegedly "the same" exhibited indeed clearly visible differences that would not have lead an examiner to an erroneous identification. There simply was no duplication of individual ridge detail in prints from different digits.
At he time when fingerprint evidence was first admitted by courts, such empirical evidence or experience in dealing with millions of fingerprint records was not available. If the courts at that era which confronted fingerprint identification evidence first [in Argentina (1892), India (1897), France (1902), and subsequently in England and the United States] had been required to satisfy a Daubert-like decision, perhaps fingerprint identifications would not have been deemed admissible in those early years. With the data that is available today, however, it would be rather ludicrous to argue that the premises underlying fingerprint identification have not been scientifically validated in the face of the accumulated experience of the millions of fingerprints that have been scrutinized by experts.
But there is, today, an opportunity to prove the underlying principle of individuality empirically in a manner that was not available in the past. The tremendous computer data bases holding millions of individual finger impressions can today be searched to determine whether pattern duplication exists. While experience has dictated such research is not necessary, the purists (or skeptics) could be satisfied by a rather simple research program that asks our AFIS systems to search, say, a partial individual print of a known person and compare the print against the entire data base. Competent fingerprint examiners feel confident that when the "statistical matches" the computer is bound to throw up are visually examined by them for concordance of individual ridge detail, no two prints from different digits will be found to match.
1 Prof. Moenssens is the author of the books Fingerprint Techniques (1971) and of Fingerprints and the Law (1969), now out of print but being readied for updated reissue as a single volume Fingerprint Identification: Techniques and Evidence. He is also the senior co-author of Scientific Evidence in Civil and Criminal Cases, now in its fourth edition.
2 And also on the soles (plantar surfaces) of the feet.
3 The most common of those classification systems were (1) the Henry System and its subsequent modifications, used widely in countries that have the common law of Great Britain as the basis for their legal systems; and (2) the Vucetich System and its derivatives, used elsewhere.
4 Automated Fingerprint Identification System.
