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SUMMARY:Prior Probability and the Presumption of Innocence - Christian Dah
 lman (Lund University)
DTSTART:20160929T123000Z
DTEND:20160929T131500Z
UID:TALK67706@talks.cam.ac.uk
CONTACT:INI IT
DESCRIPTION:My talk will address a problem of fundamental importance for t
 he Bayesian  approach to evidence assessment in criminal cases. How shall 
 a court\, operating  under the presumption of innocence\, determine the pr
 ior probability that the  defendant is guilty\, before the evidence has be
 en presented? I will examine some  ways to approach this problem\, and rev
 iew different solutions. The  considerations that determine the prior prob
 ability can be epistemic or  normative. If they are purely epistemic\, the
  fact that the defendant has been  selected for prosecution must be consid
 ered as evidence for guilt\, and this  violates the presumption of innocen
 ce (Dawid 1993\, 12). The prior probability  must therefore be determined 
 completely or partly on normative grounds. It has  been suggested by Denni
 s Lindley and others that the prior probability shall be  determined as 1/
 N\, where N is the number of people who could have committed the  act that
  the defendant is accused of (Lindley 1977\, 218\; Dawid 1993\, 11\; Bende
 r  & Nack 1995\, 236)\, but there are several objections to this solution.
  As  Leonard Jaffee has pointed out\, the prior probability will not be eq
 ual in all  criminal trials\, as N will vary from case to case (Jaffee 198
 8\, 978). This is  problematic since the doctrine of fair trial requires t
 hat defendants are  treated equally. Furthermore\, the court will not have
  sufficient knowledge about  all possible scenarios to determine N with th
 e robustness required by the  standard of proof (Dahlman\, Wahlberg & Sarw
 ar 2015\, 19). My suggestion for  the problem is that the prior probabilit
 y should be determined completely on  normative grounds\, by assigning a s
 tandardized number to N\, for example N = 100.  If the number of people wh
 o could have committed the crime is always presumed to  be 100\, the proba
 bility that the defendant is guilty before the evidence has  been presente
 d will be 1% in all trials. According to this solution\, the prior  probab
 ility is an institutional fact (Searle 1995\, 104).
LOCATION:Seminar Room 1\, Newton Institute
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