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15 Psychol. Pub. Pol'y & L. 315 (2009)
Prime Suspect: An Examination of Factors That Aggravate and Counteract Confirmation Bias in Criminal Investigations

handle is hein.journals/psypbclw15 and id is 351 raw text is: 

Psychology, Public Policy, and Law                  @ 2009 American Psychological Association
2009, Vol. 15, No. 4, 315-334                     1076-8971/09/$12.00 DOI: 10.1037/a0017881

                     CONFIRMATION BIAS IN

                                Barbara  O'Brien
                     Michigan  State University College of Law

   Confirmation bias is the tendency to bolster a hypothesis by seeking consistent
   evidence while minimizing inconsistent evidence. In criminal investigations, pre-
   ferring hypothesis-consistent information could undermine accuracy by leading
   investigators to disregard evidence that challenges their theory of a case. Two
   studies examine factors that influence confirmation bias in criminal investigations.
   In Study 1 (N = 108), participants who articulated a hypothesis early in their review
   of a mock police file showed bias in seeking and interpreting evidence to favor that
   hypothesis. In Study 2 (N = 109), participants who considered why their hypothesis
   might be wrong showed less bias, but those who generated additional hypotheses did
   not. Implications for improving accuracy of investigations and suggestions for
   future research are discussed.
   Keywords: confirmatory bias, confirmation bias, criminal justice, law

   Whenever a false   conviction  comes  to light, the obvious question is how the
jury got it so wrong. How did 12 impartial people find beyond  a reasonable doubt
that an innocent  person  was  guilty? Yet false convictions reflect more  than  a
jury's mistake; they reflect mistakes by police and prosecutors. Police suspected
an  innocent person  and the prosecutor judged  the case  to be strong enough   to
warrant  filing charges. The mistakes  that landed an  innocent person  in prison
began  long before the jury heard the first witness.
    With  dramatic  failures, we tend to look for dramatic causes  like deliberate
misconduct   or gross negligence.  Nevertheless, false convictions  often involve
honest  mistakes by  ethical investigators and prosecutors. Estimating how  often
such errors occur is difficult because we only know about the errors we catch, but
those cases  in which  defendants have  been  exonerated  provide a  window   into
what  can go wrong  in an investigation. Gross and  colleagues (2005) studied  the

    These studies were conducted as part of the author's doctoral dissertation at the University of
Michigan. Many thanks to Richard Gonzales, J. Frank Yates, Samuel R. Gross, Norbert Kerr,
Richard E. Lucas, and especially, Phoebe C. Ellsworth, for guidance and comments. Several
excellent research assistants from the University of Michigan Department of Psychology assisted in
data collection and coding: Andrea Rice, Zach Sawaya, Megan Ferkel, Chelsea Goforth, Hannah
Fishman, and Jesse Smith. The author is indebted to the superb research librarians at the Michigan
State University College of Law for their help. During preparation of this article, the author was
supported by a doctoral fellowship from the University of Michigan Department of Psychology, and
a summer research grant from the Michigan State University College of Law. This research was
previously presented at the Conference on Empirical Legal Studies at the University of Texas School
of Law in November 2006.
    Correspondence concerning this article should be addressed to Barbara O'Brien, Assistant
Professor, 455 Law College Building, Michigan State University College of Law, East Lansing,
Michigan 48824; E-mail: obrienb@law.msu.edu


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