What is relative plausibility?

AuthorDavid S Schwartz,Elliott Sober
Published date01 April 2019
Date01 April 2019
DOIhttp://doi.org/10.1177/1365712718816259
Subject MatterArticles
EPJ816259 198..204 Article
The International Journal of
Evidence & Proof
What is relative plausibility?
2019, Vol. 23(1-2) 198–204
ª The Author(s) 2019
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DOI: 10.1177/1365712718816259
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David S Schwartz
University of Wisconsin Law School, Madison, USA
Elliott Sober
Department of Philosophy, University of Wisconsin, Madison, USA
Abstract
Allen and Pardo’s explanation of Relative Plausibility as a theory of evidence and proof in
litigation is ambiguous and underspecified. Their account suggests at least three different
interpretations of what they mean. They might be advocating “anti-halfism,” which tracks the
“conventional account” but merely rejects >0.5 as the proper standard of proof. Or they might
be advocating “probabilistic holism,” in which trial decision-makers apply probability to whole
claims but not elements – in which case it remains to be explained how such an approach is
internally coherent. Or they might be endorsing “total anti-probabilism,” in which “plausibility”
obeys rules and axioms different from those of probability – rules and axioms that Allen and
Pardo have yet to identify. To date, Allen and Pardo have side-stepped criticisms by shifting
from one interpretation to another, strategically. Aside from presenting a theory too formless
to determine how well it fits actual jury behavior, Allen and Pardo have not presented any
robust empirical observations about how juries actually decide cases (despite their claims to do
so). Before we can really assess whether Relative Plausibility is a new paradigm for under-
standing the structure of evidence and proof in litigation, Allen and Pardo must tell us much
more about what it actually is.
Keywords
burden of proof, conjunction problem, plausibility, probability, jury behavior
We are grateful to Ron Allen and Mike Pardo, not only for including us in this symposium of responses
to their defence of their Relative Plausibility theory, but for engaging so closely with our article, ‘The
conjunction problem and the logic of jury findings’ (Schwartz and Sober, 2017: 619). They are highly
critical of it, but in academia as elsewhere, the only thing worse than being talked about is not being
talked about. In our article, we discussed Relative Plausibility in a limited way, to point out that it does
not avoid the conjunction problem. By treating us as full-fledged critics of Relative Plausibility, Allen
Corresponding author:
David S Schwartz, University of Wisconsin Law School, 975 Bascom Mall Madison, Madison, WI 53703, USA.
E-mail: dsschwartz@wisc.edu

Schwartz and Sober
199
and Pardo have exceeded the scope of our argument, but we welcome the invitation to engage in a more
wide-ranging critique of their theory.
Allen and Pardo claim that Relative Plausibility is a new paradigm for understanding the structure of
evidence and proof in litigation, one that displaces what they call the ‘conventional account’. The
conventional account, as they see it, is an approach that employs probability theory and adopts a standard
of proof in civil cases expressed as ‘more probable than not’ or >0.5 on the standard 0 to 1 probability
scale. Allen and Pardo’s account of Relative Plausibility has three elements: a purported refutation of
probability theory as inapplicable to trial evidence, a description of Relative Plausibility, and an ambi-
tious empirical claim that Relative Plausibility better fits the observable evidence about jury behaviours.
We don’t have the space here to mount an extended defence of probability theory, but we note that even
if Allen and Pardo could demonstrate that probability theory is incoherent as applied to trial evidence
questions, it does not follow that Relative Plausibility is itself a coherent alternative.
Allen and Pardo’s explanation of Relative Plausibility is ambiguous and underspecified, suggesting at
least three different interpretations of what they might mean. The ambiguity of their account allows them
to side-step certain criticisms by shifting from one interpretation to another. In this essay, we will try to
pin them down. As for their empirical argument, that too is unpersuasive. Their theory is at present too
formless to allow us to determine how well it fits actual jury behaviour, and in any case Allen and Pardo
have not presented the robust empirical observations they sometimes claim.
A. The conjunction problem
Allen and Pardo’s attack on the coherence of probability theory as applied to trial fact-finding relies
heavily on the conjunction problem. In our article, we argued that the conjunction problem does not
seriously undermine the standard probabilistic account of proof at trial and therefore doesn’t provide a
good reason for rejecting that account in favour of an alternative, such as Relative Plausibility. The
conjunction problem is not an inherent problem within the probabilistic account of proof, but is created
by a particular interpretation of jury instructions that says that a plaintiff wins whenever she proves each
element of her claim is more probable than not, interpreted as Pr(X)>0.5, where X is the plaintiff’s claim
and Pr(X) þ Pr(not-X) ¼ 1. The conjunction problem occurs when the plaintiff meets the ‘each element
>0.5’ condition but fails to meet the intuitive and legally mandated ‘whole claim >0.5’ condition due to
the multiplication rule for conjunctions. (Cases where the plaintiff meets neither condition or both
conditions are unproblematic.) But, we argued, no practical problem arises unless there are many...

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