The limitations of relative plausibility theory
| Author | Dale A Nance |
| DOI | 10.1177/1365712718816256 |
| Published date | 01 April 2019 |
| Date | 01 April 2019 |
| Subject Matter | Articles |
The International Journal of
Evidence & Proof
The limitations of relative
2019, Vol. 23(1-2) 154–160
ª The Author(s) 2019
plausibility theory
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DOI: 10.1177/1365712718816256
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Dale A Nance
Case Western Reserve University, Cleveland, USA
Abstract
The relative plausibility theory advanced by Professors Allen and Pardo is an important con-
tribution to understanding the process of drawing inferences from evidence. It is, to be sure, an
incomplete descriptive theory, and it suffers from defects as an interpretive theory. These
problems are even more acute when they offer the theory as a model of the burdens of proof
and the full panoply of rules related thereto.
Keywords
burden of proof, inference, probability, plausibility, explanation
It is indeed a pleasure to contribute to a discussion of ‘Relative plausibility and its critics’, by Ron Allen
and Michael Pardo (hereafter, ‘A&P’) (Allen and Pardo, 2019). The theory they have advanced is an
important contribution to understanding inferences from evidence at trial. But it is not the whole story.
And there are numerous problems to be addressed in comparing it with a probabilistic interpretation of
the standards of proof. Unfortunately, in the limited space I have, I can address only a few important
points, and each only briefly.1
The domain(s) of inquiry
A&P compare their relative plausibility theory with what they consider the conventional probabilistic
account. The latter includes two, quite distinct theories: a theory of inference, based on processes (not
necessarily calculations) conforming to the rules of mathematical probability, such as Bayes’s Rule
(hereafter, ‘Bayesian inference’); and a theory of the burdens of proof based on conventional probabil-
istic decision theory (hereafter, ‘decision theory’). As often done before, A&P proceed as if these two are
inextricably connected (they are not) and as if the former only makes sense as involving serial updating
calculations (it does not). Besides the mischaracterisation of Bayesian inference models, this ignores the
fact, noted by several scholars, that decision theory can be used as a model of proof standards regardless
1. My views on the matter are discussed at length in Nance (2016).
Corresponding author:
Dale A Nance, School of Law, Case Western Reserve University, Cleveland, Ohio, 44106, USA.
E-mail: dale.nance@case.edu
Nance
155
of how fact-finders reach their assessments of likelihood that a claim is true.2 So long as they can capture
their overall assessment of the comparative strength of the evidence in the form of an approximate odds
ratio (e.g., the evidence favours the claimant over the defendant by a ratio of at least n:1), decision theory
can be employed, assuming that other objections to such a theory can be overcome. One can give, and I
have given, a qualified endorsement both to the decision-theoretic model of standards of proof and to
relative plausibility as a theory of factual inference—without inconsistency. A&P’s presentation of their
theory as providing the key to both domains of inquiry, however, places a greater burden on their theory
than it can bear.
The task at hand
A&P characterise their task as that of articulating the best explanation of juridical fact-finding. At other
times, they describe their enterprise as descriptive, as if an explanation is a description. I think of these as
different. The best description of the orbit of a planet may be that it moves in an ellipse around the sun,
but the best explanation of that movement would involve recourse to a theory of gravity. Differentiating
their theory from the so-called ‘story model’, A&P say their theory is not a ‘psychological’ theory of
behaviour, thus opening a potential gap between accurate description and whatever it is that they are
doing (see, e.g. Allen and Pardo, 2019: 17 n 86). Acknowledging that their theory does not account for
all the details of the various litigation systems within even the American branch of the common-law
tradition, they maintain that their theory is an effort ‘to capture to the extent possible the essence of the
object under inquiry’, that is, juridical proof (see Allen and Pardo, 2019: 7). As juridical proof is a
purposive activity, a variety of normative issues inevitably pervade such a theory of ‘essence’, and
indeed A&P compare the competing theories in terms of how they fit the goals of adjudication. For this
reason, I think they are actually offering an interpretation of the practices that constitute the object of
inquiry, for which both descriptive accuracy and normative appeal have a role.3
In any event, A&P emphasise many ways in which their account is consistent with prevailing
litigation practices. I mention this only because, when addressing other scholars’ claims that aspects
of probabilistic theory are consistent with legal norms, A&P are inclined to be dismissive, saying things
like ‘consistency is not enough’.4 One cannot have it both ways: either consistency (or lack thereof)
counts in favour of (or against) the descriptive component of an interpretation, or it does not. The answer
is that is does, though of course that is not the whole story.5 To be sure, from the way A&P articulate this
particular...
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