A dark and stormy night: the mystery of the missing science in fingerprint identification.

• Author: Schmidt, Dorothy E.

IN MAY of 2004, the FBI issued an unprecedented apology to Brandon Mayfield, the Portland, Oregon lawyer who was tied to the Madrid train bombings via an erroneous fingerprint identification. (1) The print in question had been positively identified as Mayfield's by three highly qualified FBI examiners and confirmed by Mayfield's own expert. Yet it was ultimately shown to belong to a far more plausible suspect, an Algerian named Ouhnane Daoud, who was linked to the bombings through other evidence. (2) Especially perplexing to the casual observer was the fact that Mr. Mayfield was arrested even though the fingerprint match had been disputed by police in Spain. An even greater mystery, however, was raised by Dr. Simon Cole, Professor of Criminology at U.C. Irvine, who was quoted in news coverage of the apology as stating that fingerprinting methodology has never been scientifically validated. (3)

This article argues that fingerprint methodology should become more science-based in the future because the adversary system is not structurally suited to prevent or correct the types of mistakes that led to the Mayfield fiasco. (4) It is already slowly evolving in that direction, in response both to the pressures created by those same mistakes, and to the evolving relationship between courts and scientific evidence in general. Additionally, defense attorneys should adopt a more nuanced approach to challenging fingerprints than they have been doing if they want judges to take them seriously.

1. Setting the Stage: Colonial India to Illinois

   Fingerprints have a deep history of use for personal identification. A Persian historian wrote in 1303, "Experience shows that no two individuals have fingers precisely alike." (5) This statement illustrates two highly significant aspects of fingerprints: anyone can see their patterns without special training or instruments, and there is strong intuitive appeal in the conclusion that they are unique to the individual. This section considers how the history of fingerprint identification in Western forensics shaped its current status in the U.S.

   1. Record Keeping in the British Empire

      The British Empire first became interested in forensic applications of fingerprints in the 1800's, in connection with identifying repeat offenders. William Hershel began to collect fingerprints as a hobby in 1859, and while serving as a colonial law enforcement administrator in India in the 1870's he had the opportunity to employ his experience attempting to identify recidivist criminals. (6) The obvious drawback to this system in the pre-computer age was the prohibitively labor-intensive nature of comparing each new print set with numerous previous sets to try and find a match. (7) At about the same time, fellow British colonial Henry Faulds observed that potters left identifiable fingerprints on their clay wares, and he was inspired to consider their potential for personal identification. Faulds was a physician in a hospital in Tokyo when he reportedly solved a petty crime using "greasy finger-marks." Faulds's early role is significant because he raised concerns about the potential for false identification. He examined thousands of sets of ten prints, and became convinced no two were alike, but he also believed systematic study should be used to determine whether single prints were also unique. Notably, the rest of the fingerprint community was less enthused about undertaking such a study.

      When you glance at your own fingerprints, you see the first level of detail of the pattern, i.e., whether it is a loop, an arch, a whorl, etc. The idea of matching sets of ten prints to already-convicted criminals for the purpose of identifying repeat offenders has great intuitive appeal, particularly when you look closer and begin to pick out the second level of detail, the way individual ridges begin and end and bifurcate to create a wealth of easily identifiable features. Two questions in the current debate are the most fundamental ones addressed by Faulds and Hershel: (1) Is every print unique? and (2) Do prints change over time? Both of the esteemed Englishmen convinced themselves through personal observation that fingerprints remain stable over time. Concerning the first question, however, the science community of their day closed ranks around the conclusion that every print is unique, although it is virtually impossible to demonstrate. If I look at 100 people's prints every year for a decade, and never see a change (except scarring), there's no reason to suppose that adding another 1,000 or 100,000 people will yield a different result. Fingerprints change or they don't, yet how does one conclusively "prove" that no two are alike, without physically comparing every print on the planet?

      Before we turn to how fingerprints gained acceptance in U.S. courts, it will be useful to consider the first documented case in British law where fingerprints were used as evidence in a murder trial. In 1898, in Bengal, a man's former servant was released from prison, and began spending large sums of money whose source he could not explain. The former master was found murdered, and a single bloody fingerprint left at the scene was introduced into evidence. (8) Since there were no "fingerprint experts" yet, the print was examined by the judge and assessors, who convinced themselves it must belong to the suspect. First, to prove absolutely that the print was unique would have required fingerprinting every other person on earth. Second, the higher the quality of partial print(s) taken from crime scenes, in size and clarity, the higher the probability that some obvious discrepancy will be found, leading to exclusion--a positive non-match. The poorer the quality of the print in evidence, the greater the subjectivity of any human assessment, and the greater the chance of erroneous conclusions of a positive "match." More ominously, if you believe fingerprints are unique, the more different-from-all-others you believe each one is, the less information you think you need for a positive match. This creates a powerful disincentive to delve further into the subject of uniqueness. Faulds, a physician, campaigned long and unsuccessfully for systematic verification of just how unique human fingerprints may be. For his trouble, he was effectively shut out of the fingerprint community. (9)

   2. Rolling Over in the States

      It didn't take long for the new science of fingerprint identification to cross the Atlantic. In 1911, the Supreme Court of Illinois became the first state high court to declare evidence derived from the technique of matching fingerprints to be admissible, in People v. Jennings. (10) This landmark case is analyzed in detail by Simon Cole in a paper focused on "the nitty-gritty of expert [fingerprint] testimony." (11) In Jennings, no less than five fingerprint examiners, all of whom were trained by a Scotland Yard detective, agreed that fingerprints left at a murder scene "matched" the defendant's rolled prints. One of them even testified, in response to a question as to whether it was his "opinion" that the prints were a match, "I am positive. It is not my opinion." (12) This response foreshadowed an approach to fingerprint testimony which has prevailed for ninety-five plus years. Absolute certainty is presumed, thereby implying infallibility of the expert testimony.

      The defense argued that the expert's testimony was unnecessary, on the grounds that "the jury can take the magnifying glass and then come to a conclusion as to whether or not these photographs are identical." (13) The court concluded otherwise, stating flatly that, "the classification of finger print impressions and their method of identification is a science requiring study." (14) This case also set the stage for another feature of the present debate concerning fingerprint evidence, specifically, whether or not it is a "science." As Cole notes, evidence that convincingly implies that "only one person could be the source of a piece of trace evidence" is an enormously powerful tool for the prosecution. (15)

      Shortly after Jennings, a New York case illustrated the power of fingerprint evidence under this approach. In People v. Crispi, a fingerprint was the only evidence against a defendant whose family provided an alibi, contrasted with Jennings, where the fingerprint supplemented strong circumstantial evidence. (16) As in Jennings, the defense attacked the need for expertise, but the court disagreed, calling the technique a "new science." (17) As Cole observes, "fingerprint evidence benefited from its position on the cusp between scientific and lay knowledge.... A fingerprint ... was commonplace enough to be readily understood by the jury, yet sufficiently esoteric to require expert interpretation." (18) Crispi's attorney challenged the fingerprint expert by pointing out a discrepancy between the latent and rolled prints, to no avail. The expert declared the two prints "identical," explaining, "[y]ou must take into consideration that the prints may be under different conditions, more pressure being used." (19) This illustrates a feature of the fingerprint art which remains true today, that such an expert "walks a thin line between transparency and opacity, between science and common sense." (20) In theory the Crispi jurors could have disregarded the fingerprint testimony, but in fact they probably "lacked the epistemological authority for doing so." (21) Following the fingerprint testimony, the hapless Crispi pled guilty, ending the trial.

   3. The Chimera: Is it Science, Art, or Something Else Entirely?

      One further aspect of fingerprint identification is pivotal to the present status. Early practitioners "developed a novel occupational norm: unanimity." (22) For example, a 1917 instruction manual asserts that "[t]he testimony of a finger print expert is not subject to contradiction by another finger print expert." (23) In 1919, another author suggested that...