Evidence, probability, and relative plausibility: A response to Aitken, Taroni, and Bozza
| Author | Ronald J. Allen,Michael S. Pardo |
| DOI | http://doi.org/10.1177/13657127231155798 |
| Published date | 01 April 2023 |
| Date | 01 April 2023 |
| Subject Matter | Articles |
Evidence, probability, and relative
plausibility: A response to Aitken,
Taroni, and Bozza
Ronald J. Allen
*
Northwestern University Pritzker School of Law, Chicago, IL, USA
Michael S. Pardo
**
Georgetown University Law Center, Washington, DC, USA
A paradigm shift is occurring in legal epistemology, replacing probability theory as the best explanation
of juridical proof with a form of explanationism that has come to be known as the “relative plausibility”
theory (see Allen and Pardo, 2019a). The probabilistic paradigm saw the common law legal systems as
one large probabilistic event best explainable by reference to the probability calculus. The primary suc-
cesses of that approach were to offer widely accepted explanations of burdens of persuasion as probabil-
istic thresholds and the meaning of the legal concepts of relevance and probative value. After a
remarkable burst of enthusiasm reached its apex in the latter part of the twentieth century, difficulties
began to be perceived—irritants in the language of Thomas Kuhn—that could not easily be accommo-
dated within the reigning paradigm. As Kuhn observed and predicted, painting on a much larger and more
important canvas than legal epistemology, the initial reaction of the adherents to the reigning paradigm is
to attempt to explain away irritants, but in this case the irritants also led to new theorizing about the object
of inquiry. That initial theorizing evolved over time into an explanation of common law legal systems that
preserves a place for probabilistic reasoning but is dominated by the idea that the parties create and liti-
gate alternative explanations for the events under consideration. The fact-finder (judge or jury) weighs
their various merits and decides between them, or in their light creates the fact-finder’s own explanation
of what happened—and that explanation wins the day.
1
* John Henry Wigmore Professor, Northwestern University Pritzker School of Law, President, International Association of
Evidence Science; Fellow, The Forensic Science Institute, China University of Political Science and Law. We are indebted to
Christian Dahlman for comments on an earlier draft of this article.
** Professor of Law, Georgetown University Law Center.
Corresponding author:
Ronald J. Allen, Northwestern University Pritzker School of Law, Chicago, IL, USA.
Email: rjallen@law.northwestern.edu
1. In civil cases, this involves a direct comparison between the parties’explanations; in criminal cases, a reasonable doubt exists if
the prosecution does not have a plausible explanation, or the defendant does have one even if less plausible than the government’s
explanation.
Article
The International Journal of
Evidence & Proof
2023, Vol. 27(2) 126–142
© The Author(s) 2023
Article reuse guidelines:
sagepub.com/journals-permissions
DOI: 10.1177/13657127231155798
journals.sagepub.com/home/epj
The relative plausibility theory seems to be winning the day, although as Kuhn observed, the adherents
to the replaced paradigms do not give up easily; nor should they. After all, the old may be better than the
new. Various efforts to rescue the probabilistic paradigm have been constructed, but the consensus seems
to be growing that the new is better than the old.
2
Nonetheless, three distinguished forensic scientists have
recently amounted yet another defense of a version of probabilism. Colin Aitken, Franco Taroni, and
Silvia Bozza (henceforth ATB) have recently announced and attempted to justify that “The only way
to combine uncertainties about events or different items of evidence is to use the rules of probability,”
(Aitken et al., 2022: 311) and as consequence, rather than relative plausibility, “probabilistic reasoning
remains the best way in which to evaluate and interpret evidence”(Aitken et al., 2022). And the way to do
that is through employing a theorem of conventional probability theory, Bayes’Theorem. We are grateful
to ATB for their careful attention to our work, and for the opportunity that it provides to both clarify
aspects of relative plausibility and to explore some issues in greater depth.
To begin, both the announcement and the central propositions derived from it are, shall we say, highly
improbable. At the same time, there is much at a more discrete level that is quite valuable in ATB’s ana-
lysis. We explain both why their defense of the probabilistic paradigm is unconvincing and point out
some of the valuable aspects of their analysis.
The first thing to note is that the foci of ATB and the relative plausibility theory are quite different.
ATB focuses exclusively on evidentiary evaluation—on how a fact-finder should go about deciding if
a material proposition is true.
3
They try to explain, as others have, how Bayes’Theorem may describe
and prescribe fact-finder evaluation of evidence, and how some of the anomalies in the probabilistic para-
digm can be resolved. These are important issues, but they are only part of the relative plausibility
explanation:
relative plausibility is not only about jury decision-making and the decision rules at trial. Its scope is much
broader. It is about the entire process of proof, including (1) the form, securing, and presentation of evidence,
(2) the forms of argumentation employed at trial, (3) the manner in which humans process and deliberate on
evidence, (4) the trial structure created by the rules of evidence and procedure, (5) the structure of litigation
before and after trial, (6) the manner in which judges and juries, on the one hand, and trial and appellate judges,
on the other hand, interact, and (7) to some extent, the meaning and nature of rationality. In other words, all of
these features comprise the sprawling entity that we refer to as ‘juridical proof’, and relative plausibility is an
attempt to explain that entity in all of its aspects, from beginning to end.
4
ATB could be correct about everything that they say and yet the relative plausibility theory may
remain the best explanation so far of juridical proof.
5
They are not, however, right about everything
that they say, including and especially the very foundations of their argument.
2. As to the extent of the revolution, see Allen and Pardo (2019a). Sixteen of the twenty commentators on the original article, who
were selected to be critics, more or less agreed that this shift is occurring. The remaining four express uncertainty about the
matter. See Doak (2019).
3. Aitken et al. (2022: 312) (“the choice of probability as a framework for reasoning under uncertainty is not made to suggest that it
provides a model for the working of the legal process as a whole.”)
4. Allen and Pardo (2019b: 205, 207–208d). The different foci of the two enterprises are captured by ATB’s assertion that “The
concept of explanationism has been developed because of apparent analytic difficulties detected in the probabilistic paradigm”
(Aitken et al., 2022: 311). This confuses origins with evolution. The “analytic difficulties”were responsible for focusing attention
in a sustained way on juridical proof, but analysis of it has moved beyond those problems.
5. We are not quite certain about what they actually are saying. At times they make very broad statements, such as probabilistic
reasoning is the best way “to reason coherently in the presence of uncertainty,”(Aitken et al., 2022: 311); “probabilistic reason-
ing remains the best way in which to evaluate and interpret evidence”including legal evidence (ibid.: 311); “The only way to
combine uncertainties about events or different items of evidence is to use the rules of probability,”(ibid.: 311); and “probabil-
ities are subjective beliefs,”(ibid.: 321). At other times they seem to be making quite limited statements, such as:
Allen and Pardo 127
To continue reading
Request your trial