Presenting evidence in court

Presenting evidence in court–some fundamental problems to
be considered
Anders Eriksson
Department of Philosophy, Linguistics and Theory of Science, University of Gothenburg, Sweden
anders.eriksson@ling.gu.se
The question of how to present evidence in court has been a hotly debated question for some
years now. One may identify the Daubert ruling 1 as a possible starting point, but it is likely that
even without Daubert, these questions would still have emerged. We have had some
presentations at our own meetings too on this topic, most notably a couple of presentations by
Geoffrey Morrison advocating the Likelihood Ratio approach. But it is now time to take a
broader grip on these questions. There are many more aspects that just the pros and cons of the
Likelihood Ratio approach to be considered.
The ongoing discussions have resulted in two important surveys, one in the US and one in the
U.K.. The results of these surveys have been published in two recent reports – Strengthening
Forensic Science in the United States (2009) published by the National Research Council and
Expert Evidence in Criminal Proceedings in England and Wales (2011) published by The Law
Commission. Both reports have a strong focus on the question of admissibility as may be
expected, but most of what is said is nevertheless highly relevant also in those countries where
the concept of admissibility does not exist within the legal system. It is rather obvious in these
reports and many other publications that the authors see DNA analysis as the role model for all
types of evidence.
With the exception of nuclear DNA analysis, however, no forensic method has been
rigorously shown to have the capacity to consistently, and with a high degree of certainty,
demonstrate a connection between evidence and a specific individual or source (National
Research Council report, p. 7)
Whether it is possible or even desirable to squeeze all types of evidence into this model is
something you might want to consider. But even if that should in principle be possible we are
very far from that today and then the question arises how the strength of different types of
evidence can be evaluated and compared. This is by no means obvious even if we limit the scope
to the field of forensic phonetics. And perhaps even more problematic is the case when different
types of evidence are contradictory. I will describe a case (Donnelly, 2005) where eyewitness
testimony contradicted DNA and how this caused almost total confusion.
The Daubert ruling by the US Supreme Court meant a substantial change away from previously
accepted admissibility rules defined by the Frye ruling 2 . The Frye ruling meant that admissible
evidence should be based on principles generally accepted within the relevant scientific
community, in principle meaning consensus. The Daubert ruling replaced the idea of scientific
consensus by a set of more general criteria meant as a guide but not as “a definitive checklist or
test”.
1 Daubert v. Merrell Dow Pharmaceuticals, Inc, 509 US 579 (1993)
2 Frye v. United States, 293 F. 1013 (D.C. Cir. 1923)

1) Is the evidence based on a testable theory or technique;
2) has the theory or technique been peer reviewed;
3) in the case of a particular technique, does it have a known error rate and standards controlling
the techniques operation;
4) is the underlying science generally accepted
As a general guide these rules make a lot of sense, stressing scientific validity and the
competence of the expert witness, but without requiring consensus in the scientific community.
The problematic part is, however, who is to be the judge if the evidence complies with these
rules. In the US where Daubert has now been applied for some years the final word concerning
admissibility seems to have shifted from the scientific community to the trial judge who in most
cases is likely to possess little or no competence to evaluate scientific validity and reliability. As
may be expected this has not been without problems as I will explain.
Finally, I will say a few words about individualization. The problem of individualization has a
long history. One formulation of what it means for entities to be identical goes by the name of
The Identity of Indiscernibles often referred to as Leibniz's law. Leibniz first formulated the
principle in his Discourse on Metaphysics (1686): "it is not true that two substances may be
exactly alike and differ only numerically". In logical terms the law is often re-formulated as: if,
for every property F, object x has F if and only if object y has F, then x is identical to y. This is
also by and large the way we think about identity in forensic speaker identification although we
never come even close to the "for every property" requirement in any analysis. Two questions
arise when we thing about this law: 1) Is it true even in principle and 2) If it were can we even
approach fulfilment of the requirements by any practically applicable methods in forensic
science. Some scientist have argued that the principle is simply false, at least for some areas of
Quantum Physics (Cortes, 1976). Others (e.g. Saks & Koehler, 2008) have attacked the principle
on more practical grounds. Based on these and other studies, I will argue that we should abandon
the concept of identity altogether in the forensic context.
References
Cortes, A. (1976). Leibniz's principle of the identity of indiscernibles: A false principle. Philosophy of
Science, 43(4): 491–505.
Donnelly, P. (2005). Appealing statistics. Significance, 2(1), 46–48.
National Research Council. (2009). Strengthening Forensic Science in the United States: A Path Forward.
Washington D.C.: The National Academies Press.
Law Commission. (2011). Expert Evidence in Criminal Proceedings in England and Wales. (LAW COM No
325)
Leibniz, G. W. (1686). Discourse on Metaphysics. (on-line translation by G. R. Montgomery.
http://www.anselm.edu/homepage/dbanach/Leibniz-Discourse.htm)
Saks, M. J. and J. J. Koehler (2008). The individualization fallacy in forensic science evidence. Vanderbilt
Law Review, 61(1): 199–219.