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12 Law & Hum. Behav. 1 (1988)

handle is hein.journals/lwhmbv12 and id is 1 raw text is: Law and Human Behavior, Vol. 12, No. 1, 1988

Bayes' Theorem in the Trial Process
Instructing Jurors on the Value of Statistical Evidence*
David L. Faigman and A. J. Baglioni, Jr.t
The use of statistics and probabilities as legal evidence has recently come under increased scrutiny.
Judges' and jurors' ability to understand and use this type of evidence has been of special concern.
Finkelstein and Fairley (1970) proposed introducing Bayes' theorem into the courtroom to aid the
fact-finder evaluate this type of evidence. The present study addressed individuals' ability to use
statistical information as well as their ability to understand and use an expert's Bayesian explanation
of that evidence. One hundred and eighty continuing education students were presented with a tran-
script purportedly taken from an actual trial and were asked to make several subjective probabiliy
judgments regarding blood-grouping evidence. The results extend to the trial process previous psy-
chological research suggesting that individuals generally underutilize statistical information, as com-
pared to a Bayesian model. In addition, subjects in this study generally ignored the expert's Bayesian
explanation of the statistical evidence.
Statistics and probabilities are receiving increased attention in the law. Since 1960
there has been a dramatic growth of cases using some form of statistical evidence,
with the greatest surge coming in the late 1970s (Fienberg & Straf, 1982; Note,
1983). In light of this increased use in the courtroom, legal scholars have begun to
debate the merits of various forms of statistical evidence. These commentators
have been especially concerned with judges' and jurors' ability to understand and
use this evidence. One proposal that has garnered much attention in this debate is
* This study was conducted in fulfillment of the first author's requirement for a Master of Arts degree
at the University of Virginia. A version of this paper was presented at the 1983 American Psycholog-
ical Association convention in Anaheim, California. The authors would like to thank John Monahan,
N, Dickon Reppucci, Lois A. Weithorn, and Timothy D. Wilson for comments on earlier drafts of
this paper.
t University of Virginia.

0147-7307/88/0300-0001$06.00/0 © 1988 Plenum Publishing Corporation

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