Beyond Uniqueness: The Birthday Paradox, Source Attribution, and Individualization in Forensic Science Testimony

David H. Kaye

Pennsylvania State University, Penn State Law

November 5, 2012

Law, Probability & Risk, November 2012

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.

Number of Pages in PDF File: 13

Keywords: evidence, forensic science, uniqueness, individualization, source attribution, likelihood ratio, fingerprints, toolmarks, birthday paradox

JEL Classification: C11, C12, C19

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Date posted: November 7, 2012  

Suggested Citation

Kaye, David H., Beyond Uniqueness: The Birthday Paradox, Source Attribution, and Individualization in Forensic Science Testimony (November 5, 2012). Law, Probability & Risk, November 2012. Available at SSRN: http://ssrn.com/abstract=2172388

Contact Information

David H. Kaye (Contact Author)
Pennsylvania State University, Penn State Law ( email )
Lewis Katz Building
University Park, PA 16802
United States

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