Thursday, November 22, 2012

So What If Every Fingerprint Is Unique?

David Kaye, Beyond Uniqueness: The Birthday Paradox, Source Attribution, and Individualization in Forensic Science Testimony, Law, __ Probability and Risk 1-9 (Advance Access, Nov. 5, 2012) doi:10.1093/lpr/mgs031 (footnotes omitted),  :

"The fact that the surfaces of every individual’s fingers are unique (at a sufficient level of detail) is not, in itself, a persuasive argument for universal individualization.The uniqueness argument proves too much, for just as every finger presumably is unique, so is every latent print, even those from the same skin."

Abstract of Paper:

For many decades, forensic science identification experts have insisted that they can ‘individualize’ traces such as fingerprints and toolmarks to the one and only one object that produced them. They have relied on a theory of global uniqueness of patterns as the basis for such individualization. Although forensic practitioners and theorists are moving toward a more probabilistic understanding of pattern matching, textbooks and reference works continue to assert that uniqueness justifies individualization and that experience demonstrates discernible uniqueness. One response to the last claim applies a famous problem in probability theory—the Birthday Problem—to the forensic realm to show that even an extensive record of uniqueness does little to prove that all such patterns are unique. This essay describes the probabilistic reasoning and its limits. It argues that the logic of the Birthday Paradox does indeed undercut the theory of global, general uniqueness, but that the reasoning is logically compatible with opinion testimony that a specific object is nearly certain to be the source of a pattern or trace. It also notes some alternatives to categorical claims of individualization, whether those claims are based on the theory of global, general uniqueness or instead on some less sweeping and more defensible theory.



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