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9 Cardozo L. Rev. 967 (1987-1988)
Prior Probability - A Black Hole in the Mathematician's View of the Sufficiency and Weight of Evidence

handle is hein.journals/cdozo9 and id is 983 raw text is: PRIOR PROBABILITY-A BLACK HOLE IN THE
MATHEMATICIAN'S VIEW OF THE
SUFFICIENCY AND WEIGHT OF
EVIDENCE
Leonard R. Jaffee*
INTRODUCTION
The problem of setting litigational prior probabilities has ab-
sorbed the energies of some bright lights of Evidence, Statistics, Juris-
prudence, and the Philosophy of Science.' And so it must be, while
scholars hope to mathematicize the law of proof. The problem is
hyperdense.
This is my second crack at the thing. With my first, I uttered the
blasphemy that the legal sufficiency of litigational proof cannot be a
question of relative mathematical probability or a matter measurable
by the calculus of chance.2 One of my reasons was that we can't ob-
tain suitable prior probabilities.3 A couple-or-so worthy science-fans
quickly chastened me with some questions that deserve attention.4
* Visiting Professor, Washington University School of Law. Professor, University of Lou-
isville School of Law.
I E.g., L. Cohen, The Probable and the Provable (1977); Bias, Myers & Murphy, Theoret-
ical Underpinnings of Paternity Testing, in Inclusion Probabilities in Parentage Testing
(American Ass'n of Blood Banks 1983); Brilmayer & Kornhauser, Review: Quantitative Meth-
ods and Legal Decisions, 46 U. Chi. L. Rev. 116 (1978); Kaye, The Laws of Probability and
the Law of the Land, 47 U. Chi. L. Rev. 34 (1979). Soon after I began drafting this paper,
Boston University held a probability-and-proof symposium involving notables in several disci-
plines (e.g., Charles Nesson, of Harvard, Richard Lempert, of Michigan, and William Twin-
ing, of University College, London, England). See Symposium, Probability and Inference in
the Law of Evidence, 66 B.U.L. Rev. 377 (1986). The topics included the problem of prior
probability. See id.; Cohen, Subjective Probability and the Paradox of the Gatecrasher, 1981
Ariz. St. L. J. 627; Kaye, Paradoxes, Gedanken Experiments and the Burden of Proof: A
Response to Dr. Cohen's Reply, 1981 Ariz. St. L. J. 635 (both discussing prior probabilities).
2 Jaffee, Of Probativity and Probability: Statistics, Scientific Evidence, and the Calculus of
Chance at Trial, 46 U. Pitt. L. Rev. 925 (1985).
3 Id. at 973-78, 981-93, 1008-09, 1013-16, 1017-35, 1044-46.
4 One question-whether N (where N is some relevant population of possibly liable enti-
ties) is a suitable prior probability-simply deserves more discussion than I gave it in my
previous work (id.). Another-whether the presumption of innocence in criminal cases is just
a species of presumption of nonliability also obtaining in civil cases-seemed virtually rhetori-
cal to me until I discovered that some prominent Evidence scholars didn't know that the case
law and statutory answer has long been affirmative. So I suppose I should demonstrate this
here, having presumed not to do so in my last piece. My correspondence and discussion with
Professor Kaye (Arizona State University College of Law) suggest further development of the
nonliability presumption's bearing upon prior probability of liability or guilt. Assumptions
that I had thought apparent seem to need exposition.

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