Jurors distort evidence to favor their tentative verdict as they move through the course of a trial

June 24, 2001

In 2 studies, students and prospective jurors exhibited 'predecisional distortion,' tilting information toward their early leader when evaluating evidence in mock trials.

WASHINGTON -- Presenting further proof that jurors are vulnerable to human error, psychologists at Cornell's Johnson Graduate School of Management found significant evidence of a deep bias affecting both students and prospective jurors. Kurt A. Carlson, M.S., a graduating doctoral student and J. Edward Russo, Ph.D. present their work in the June issue of the Journal of Experimental Psychology: Applied, published by the American Psychological Association (APA).

Carlson and Russo hypothesized that "predecisional distortion" of new information could cause a juror to evaluate trial evidence with a bias toward supporting whichever party that juror currently favors. Already known to sway consumer decisions, predecisional distortion would then bias juror decisions as well. Such a finding could raise questions about the adequacy of conventional jury instructions to not reach a verdict prematurely.

In their experiments, Carlson and Russo presented subjects (126 college students for a mock civil case; 122 for a mock criminal case) with packets of detailed information including case background (case facts and opening arguments) and witness affidavits. Subjects first viewed a 20-minute video that is shown to prospective jurors in the state of Wisconsin, which instructed them not to reach "hasty opinions or conclusions."

Subjects then worked their way through the materials, marking as they went along which party (plaintiff or defendant) each piece of evidence favored, checking the verdict they were leaning toward, and indicating their degree of confidence that the current leading verdict would eventually win. Finally, they were asked to reach a verdict without looking back. In a second experiment, Carlson and Russo used this methodology with 148 prospective jurors drawn from jury pools at the Kenosha County courthouse in Wisconsin.

In both experiments, results supported the hypotheses: 75 percent of students exhibited some predecisional distortion to favor the currently leading verdict, while 85 percent of prospective jurors did so. What's more, distortion increased with juror confidence in whichever verdict was currently leading.

The authors also report that, "prospective jurors in a courthouse environment exhibited about twice the magnitude of students' distortion." They believe that juror bias was greater because these individuals entered the trial with more strongly held beliefs.

The greater permanence of these beliefs led to greater confidence in their leading verdict early in the trial, which led them to exhibit greater distortion of the new evidence (to favor their currently leading party). Because they have had less exposure to the legal system than the older prospective jurors, students might not have as strongly held beliefs.

The authors conclude that predecisional distortion can exist in the courtroom just as it does in consumer or management settings. Further, actual prospective jurors showed more evidence of bias on nearly every measure when compared to students. Carlson and Russo speculate that bias may emerge because people need coherence in what they currently believe, and so they distort new evidence to resolve any conflict between that evidence and whichever party they feel is winning the trial so far. Carlson and Russo discuss at length the prospects for limiting (if not eliminating) bias, possibly through an additional pre-trial instruction to avoid even a slight leaning toward one side until all the evidence and arguments have been heard. In their paper, they cite unpublished evidence that people can avoid, or at least greatly reduce, the bias if they are made aware of it.

"Much of the work on juror decision making assumes that the evidence is evaluated without bias, and that if bias exists at all, it occurs in how evidence is weighed, or how the rule of law is applied by jurors," says Carlson. "Our work illustrates a somewhat more primitive bias." The bottom line, write the authors, "is that any form of overall judgment should be delayed as long as possible."
Article: "Biased Interpretation of Evidence by Mock Jurors," Kurt A. Carlson and J. Edward Russo, Johnson Graduate School of Management, Cornell University; Journal of Experimental Psychology - Applied, Vol 7. No.2

Until July 31, Kurt A. Carlson can be reached at the Johnson Graduate School of Management, Cornell University, kac20@cornell.edu, 607-255-4717. As of August 1, he can be reached at the Fuqua School of Business, Duke University, kurtc@duke.mail.edu, 919-660-3737.

Full text of the article is available from the APA Public Affairs Office and after June 29 at http://www.apa.org/journals/xap/xap7291.html.

The American Psychological Association (APA), in Washington, DC, is the largest scientific and professional organization representing psychology in the United States and is the world's largest association of psychologists.

APA's membership includes more than 155,000 researchers, educators, clinicians, consultants and students. Through its divisions in 53 divisions of psychology and affiliations with 60 state, territorial and Canadian provincial associations, APA works to advance psychology as a science, as a profession and as a means of promoting human welfare.

Contact: Public Affairs Office, 202-336-5700, public.affairs@apa.org.

American Psychological Association

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