The Individualization Fallacy in Forensic Science Evidence

Forensic scientists across a broad array of sub-specialties have long maintained that they can link an unknown mark (e.g., a partial fingerprint or tireprint) to a unique source. Yet no scientific basis exists for this assertion, which is sustained largely by a faulty probabilistic intuition equating infrequency with uniqueness. This Essay traces the origins of the individualization claim and explicates the various failed lines of evidence and argument offered in its support. We conclude with suggestions for improving the scientific bases of the forensic identification sciences. I. FOREWORD: THE TWO STEPS IN FORENSIC IDENTIFICATION Forensic identification science involves two fundamental steps. The first step is to compare a questioned item of evidence to an exemplar from a known source and judge whether they appear so alike that they can be said to match. The second step is to assess the meaning of that reported match: What is the probability that the questioned and the known originated from the same source?1 Different risks of error are present at each step. The risk of error in the first step is that a reported match between a questioned and a known sample might not really match. Even if the method used to compare questioned and known samples were flawless, an error could occur if, for example, one of the samples had been mislabeled or mixed up with a different sample. The risk of error associated with the second step is that, while accurate, the reported match may have arisen through coincidence and not because the samples share a common source. The risks of error at both steps affect the ultimate inferences that can be drawn about the identification evidence in a case.2 Both risks are subjects of far too little research. As to the first step, existing standards and procedures do not provide sufficient protection from erroneous conclusions that two marks are indistinguishably alike-that is, that they "match" when in fact they differ. Few, if any, criminalistics subfields have objective standards for deciding whether two patterns match. That determination is left to the judgment of each examiner. For example, consider David Stoney's discussion of fingerprint examination standards: How much correspondence between two fingerprints is sufficient to conclude that they [are the same pattern] ... ? An adequate answer ... is not currently available. The best answer at present... is that this is up to the individual expert fingerprint examiner to determine, based on that examiner's training, skill, and experience. Thus, we have an ill-defined, flexible, and explicitly subjective criterion for establishing fingerprint identification .... Any unbiased, intelligent assessment of fingerprint identification practices today reveals that there are, in reality, no standards.3 The lack of objective standards helps explain the disturbing findings from the small body of research that has been conducted on pattern matching by forensic scientists. In some tests, examiners disagreed with one another about whether various images matched.4 In others, examiners who agreed that two patterns matched disagreed (sometimes dramatically) on what constituted the match.5 Examiners differ not only in their ability to perceive pattern similarity, but also in their thresholds for declaring matches.6 Other research suggests that the match judgments of experienced criminalists are influenced by extraneous information. A study by Itel Dror et al. found that four of five fingerprint experts who previously had identified two prints as a match reached different conclusions on a later examination, after they learned that the prints were from different persons.7 In a follow-up study, six other fingerprint experts were provided with eight pairs of prints that they previously had judged.8 The study found that introduction of contextual information induced four of the six experts to change at least one of their previous match judgments. …