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Identifying And Measuring Juror Bias About Forensic Science Evidence
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Identifying And Measuring Juror Bias About Forensic Science Evidence


Lisa Smith & Ray Bull - University of Leicester

Lisa Smith & Ray Bull - University of Leicester

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  • None of these approaches have been hugely successful at predicting juror biasAuthoritarianism, JBS, and PJAQ have seen some success – but some inconsistent findings in the literature (e.g. Rape cases, gender differences, etc.)These measures may be too general to be good predictors of any case in particularNone of these take into account a juror’s perception of the evidence in a trial.
  • Oprahization – juries were reluctant to convict a defendant who had been victimised in their livesPerry Mason syndrome – belief that juries expected courtrooms to resemble scenes from this show – in particular that there was an expectation that the attorneys would manage to get witnesses/defendants to spontaneously confess
  • Robert Blake case – jurors apparently focused on the lack of GSR evidence, and felt that this should have been recovered if Blake was indeed the shooterDefendant’s effect – the Jill Dando murder trial is sometimes cited as an example of this effect – jury may have given GSR and fibre evidence more weight than it deserved
  • Producer’s effect – forensic awareness generallyPolice chief’s version – forensic awareness of criminals specifically
  • Widely believed that these issues should be addressed in pretrial hearings
  • Anti-prosecution bias – unrealistic expectations of forensic evidence in every caseMock juror task – rape trial scenario, where forensic evidence was not relevant for determining guilt**would viewers of CSI acquit for different reasons than non-viewers??Content analysis of CSI seasons 1 & 2
  • Given a case where forensic evidence would not be useful at determining guilt – issue was consent in a rape trial.Wanted to see if there was a difference in the reasons for voting not guilty between the FV and NFV groups
  • Three crime types: murder, any physical assault, any criminal caseWillingness to convict – 5 point scaleEach participant answered questions about each of the three crime typesDemographics: age, gender, education, political views, income level, etc.
  • Did not read a simulated case – only read statements and answered questionsWhy not significant in EWT condition?Seems that participants viewed EWT as strongly as forensic evidenceMixed results about demographics and the effect on willingness to convictInconsistent between circumstantial and EWT conditions
  • News media – frequent coverage of cases where forensics solved the case, or exonerated someone innocent…not likely to mention cases in which forensics are not necessary or available.Selection of task – if a CSI effect is operating, it is most likely to be doing so through schemas – so the trial scenarios should resemble cases featured in fictional representations of the CJS
  • Probative value – manipulated to produce weak/moderate/strong forensic evidence of varying typesIn the absence of case context, participants could recognise the varying probative values – but with case context they rated all strengths of evidence stronger (see graph on next slide)
  • Strong evidence – non significant effect of case contextModerate evidence – rated significantly stronger with case context, and indistinguishable from strong evidence in case context conditionWeak evidence – Only rated as ‘weak’ on the scale (e.g. below 3) when no case context. Rated significantly stronger when accompanied by case context.
  • Strong evidence – overwhelming agreement about the strength of the evidenceModerate – almost exactly the same as strongWeak – most disagreement about the strength
  • When the situation/evidence is strong and most salient, juror bias will not override common sense**e.g. Kalven & Zeisel’s Liberation HypothesisSo, if a juror bias exists about forensic evidence, it is not likely to operate in cases where the forensic evidence is of a clearly strong probative standard.This could potentially explain the disagreement in perceived strength of weak forensic evidence seen in the previous studyNew approach to CSI Effect – rather than focusing on asking people about what shows the watch, the focus here is on what beliefs they have about forensic evidence (which is what the proposed CSI Effect influences...)
  • Originally 40-ish items – derived from the anecdotal evidence in the CSI effect literature10 items were retained which had the best inter-item and item-total correlationsPCA – oblique rotation (as items were assumed to be related to one another, and therefore potentially correlated)This approach was exploratory in this research due to the fact that this is a novel scale and there is no robust empirical research to suggest that we are testing any particular hypothesis. However the items were developed using reference to the juror bias proposed by the CSI effect literature and therefore the use of principal components analysis results in an empirical confirmation of the scale’s construct validity (Lecci & Myers, 2002).
  • Component 1 – attitudes associated with the pro-prosecution CSI effect (Over estimating the ability of forensic science evidence)Component 2 – attitudes associated with the pro-defence CSI effect (high expectations of forensic evidence)
  • Used students in this study for 2 reasons:This is essentially a big pilot to test whether the FEEB component structure will come out the same in a PCAEnables us to compare a student and community sample within the thesis (methodological comment)Only the weak/moderate DNA evidence has the potential to ID the defendant (but doesn’t particularly prove guilt)The rest of the testimony was weak (e.g. EWT) or circumstantial (e.g. police)Measures: Strength of evidence ratings, probability of guilt estimates, verdict decision
  • Cronbach’s Alpha – measure of each subscale’s reliabilityCorrelations:FEEBpp / JBS – only moderately/weak correlation – both are PP biases, so seem to be related but obviously not measuring exactly the same thingFEEBpp / GBJW – small (but sig) correlationJBS / GBJW – replicates Kassin & Wrightsman studies (r = .21, p < .05)FEEBpp / FEEBpd – sig negative correlation – makes sense as one is PP and the other PD
  • Obviously a very ambiguous case (46% voted guilty)Good that DNA was the only evidence to predict probability of guilt, because it was the only evidence which had the potential to ID the defendant
  • The higher jurors scored on the FEEBpp subscale, the stronger they rated the DNA evidence as being
  • The bias is predicting the probability of guilt estimate through the effect on the perceived strength of the forensic evidenceThough the FEEBpp is a predictor directly of prob of guilt, this relationship is better described by the mediating variable (perceived strength of DNA evidence)When we take the perceived strength of evidence into account in this model, the direct effect of the FEEBpp on prob of guilt becomes less significant (meets the criteria for a partial mediation relationship)Sobel test – significant indirect effect of mediator variable on the final prob of guilt (p < .001)
  • Ultimate goal to recruit real jurors!!FEEBpd – attitudes relating to cases where evidence is expected but not heard
  • Peremptory challenges: If you are a prosecutor who decides to try a case with tenuous forensic evidence (and perhaps strong circumstantial evidence), you may wish to select jurors who score high on the FEEBpp scale (as they will overestimate the strength of weak forensic evidence)Alternatively the defence would want jurors who score low on the FEEBpp scaleAlso, use different types of forensic evidence in the scenarios – right now I am sticking with DNA as it is easy to manipulate in the scenarios (strength wise) and it keeps the studies in the thesis consistent.


  • 1. Identifying and measuring juror bias about forensic science evidence
    Jury Research Symposium – Glasgow
    Lisa L. Smith & Prof. Ray Bull
    University of Leicester
    Forensic Psychology
  • 2. Outline
    Measuring juror bias
    CSI Effect – brief look at the literature
    Current research
    When does juror bias affect decision making?
    Development of the Forensic Evidence Evaluation Bias (FEEB) scale
    Validation of the FEEB scale and mediation model
  • 3. Measuring juror bias
    Juror demographics as predictors of verdict
    Age, gender, political affiliation, ethnicity, SES
    Personality characteristics
    Authoritarianism, Locus of Control, Belief in a Just World, etc.
    Specific scales designed to measure general attitudes toward CJS
    Legal Attitudes Questionnaire (LAQ), Juror Bias Scale (JBS), Pretrial Juror Attitude Questionnaire (PJAQ)
  • 4. The CSI Effect
    Popularity of crime fiction may have an impact on jurors’ perceptions of the criminal justice system and decision making in the courtroom.
    CSI is not the first show to be blamed for jury behaviour in the courtroom (Mann, 2006)
    “Perry Mason Syndrome”
  • 5. Cole & Dioso-Villa (2007)
    Strong prosecutor’s effect
    • Jurors are acquitting in cases that lack forensic evidence
    • 6. Robert Blake case is often used as an example of this effect
    Weak prosecutor’s effect
    • Prosecutor behaviour – e.g. Explaining missing forensics, questioning jurors about viewing habits, etc.
    Defendant’s effect
    • Jurors give forensic evidence too much weight
    • 7. Jill Dando case is sometimes cited as an example
  • Cole & Dioso-Villa (2007) continued...
    Producer’s effect
    • Increased awareness about forensic science
    Professor’s effect
    • Increase in student enrolment on forensic science related subjects
    Police Chief’s version
    • Increased forensic awareness in criminals
  • Is there any proof that these effects exist?
    Support for CSI Effect based only on anecdotal claims
    Survey of 102 attorneys in Maricopa County (Thomas, 2006)
    38% reported that they had at least one trial resulting in an acquittal or hung jury because forensic evidence was not available
    80% of prosecutors believe jurors are disappointed with forensic evidence presented at trials
  • 8. First empirical research
    Podlas (2006)
    Investigated the existence of an anti-prosecution version of the CSI Effect
    N = 291 students
    Television viewing questionnaire followed by a mock juror task
    Divided participants into two groups: frequent CSI viewer (FV) and non-frequent viewer (NFV)
    Rape trial scenario
    Only issue was consent, so forensic evidence was not presented
  • 9. Results of Podlas study
    250 (86%) participants voted not guilty
    187 of these were classified as FV of CSI
    Analysis revealed no significant difference between FV and NFV groups in their reasons for voting not guilty
    Findings did not support the existence of an anti-prosecution CSI Effect
  • 10. Kim, Barak & Shelton (2009)
    N=1027 jurors
    Measured jurors’ willingness to convict without forensic evidence:
    Based on circumstantial evidence, OR
    Eyewitness testimony
    How often do you watch CSI?
    Expectations for forensic evidence
    Demographic variables
  • 11. Kim et al., continued
    Multivariate analyses, path analysis
    CSI viewing had no direct effect on willingness to convict
    CSI viewing predicted raised expectations for forensic evidence, which in turn did influence willingness to convict
    Only significant in the circumstantial evidence condition
  • 12. Limitations of previous research
    Focus on specific television programmes or genres
    Representation of forensic science in other media outlets is largely ignored (e.g. Movies, novels, news media, etc.)
    Selection of mock juror decision task
    Important to identify when a CSI effect bias is likely to impact juries, and use appropriate case scenarios in research
  • 13. Current research
    Smith, Bull, & Holliday (in press)
    Demonstrated that mock jurors were easily able to appreciate the probative value of very strong forensic evidence
    When forensic evidence was of weak/moderate probative value, case context had a significant impact on the perception of evidence strength
    There was widespread disagreement between participants about the probative value of weak DNA evidence
  • 14. The impact of case context on perceptions of the probative value of DNA evidence
    Smith, Bull, & Holliday (in press)
  • 15. Comparison of DNA strength ratings by condition
    Smith, Bull, & Holliday (in press)
  • 16. Rationale for the Forensic Evidence Evaluation Bias (FEEB) scale
    Interactionist models of jury decision making
    Juror bias will impact decision making when the evidence is of a weak or ambiguous standard
    Is it possible to identify the bias that may be operating in these cases, and use this to predict juror decisions?
    New approach to CSI effect research
    Possible use in peremptory challenges?
  • 17. Development of the FEEB scale
    Generated 40 items based on attitudes/beliefs referred to in CSI effect literature
    Final version of the scale – 10 items
    Initial study
    N = 219 jury eligible members of the public
    Completed the FEEB scale online
    Principal Components Analysis (PCA) identified two distinct components (5 items loaded on to each component)
    Explained 47.99% of the variance
  • 18. FEEB components
    Component 1
    Component 2
    If no forensics, jury should not convict
    No forensics means investigators didn’t look hard enough
    No forensics means the defendant is probably innocent
    Police should not charge without forensic evidence
    Every criminal leaves trace evidence at every scene
    Forensics always IDs the guilty person
    Forensic evidence alone is enough to convict
    Science is the most reliable way to ID perps
    Forensics always provides a conclusive answer
    Every crime can be solved with forensic science
  • 19. Validation study
    N = 159 jury eligible students
    Completed the FEEB, Juror Bias Scale (JBS; Kassin & Wrightsman, 1983), General Belief in a Just World (GBJW; Dalbert et al., 1987)
    Mock juror decision making task
    Murder trial scenario (same as in previous studies)
    Weak DNA evidence, eyewitness, police, and pathologist testimony
  • 20. Results
    PCA for the FEEB scale
    Same components as previous study
    Accounted for 58.17% of the variance
    Split the FEEB scale into two subscales
    FEEBpp – pro-prosecution items subscale (α = .82)
    FEEBpd – pro-defence items subscale (α = .81)
    Correlations between scale measures
    FEEBpp and JBS (r = .24, p < .05)
    FEEBpp and GBJW (r = .17, p < .05)
    FEEBpp and FEEBpd (r = -.53, p < .001)
  • 21. Results continued
    Overall, 73 (46%) participants voted guilty
    Those who voted guilty estimated a higher mean probability that the defendant was guilty (80.15%) compared to those who voted not-guilty (57.19%)
    What predicted perceived probability of guilt?
    Multiple regression (DNA, eyewitness, police, pathologist evidence as predictors)
    Only significant predictor was DNA evidence (β = .47, p < .001)
  • 22. Results continued
    What predicted the perceived strength of DNA evidence?
    Regression (predictors; FEEBpp, FEEBpd, JBS, GBJW)
    FEEBpp score was the only significant predictor (β = .48, p < .001)
  • 23. Proposed mediation model
    .48* (.25)
    Note: * p < .05
    Sobel test significant at p < .001
  • 24. Discussion / future research
    Predictive ability of the FEEB scale only tested on students, and only for one crime type (e.g. murder)
    Next study will recruit jury-eligible members of the public and will include a rape and robbery scenario
    FEEBpd subscale was not a significant predictor of perceived evidence strength
    Include a version of scenario with no forensic evidence presented
  • 25. Discussion/future research continued...
    Individual decision making task
    Future research should investigate this bias in the context of group deliberations
    Reducing the effect of bias
    Judges’ instructions?
    Presentation format of evidence?
    Practical use in peremptory challenges?
  • 26. References
    Dalbert, C., Montada, L., & Schmitt, M. (1987). Belief in a just world: Validity correlates of two scales. PsychologischeBeitrage, 29, 596-615.
    Cole, S., & Dioso-Villa, R. (2007). The CSI Effect: The true effect of crime scene television on the justice system. New England Law Review, 41, 435-469.
    Kassin, S., & Wrightsman, L. (1986). The construction and validation of a juror bias scale. Journal of Research in Personality, 17, 423-442.
    Kim, Y., Barak, G., & Shelton, D. (2009). Examining the CSI effect in the cases of circumstantial evidence and eyewitness testimony: Multivariate and path analyses. Journal of Criminal Justice, 37, 452-460.
    Mann, M. (2006). The CSI Effect: Better jurors through television and science? Buffalo Public Interest Law Journal, XXIV.
    Podlas, K. (2006). The CSI Effect: Exposing the media myth. Fordham Intellectual Property, Media & Entertainment, 16, 429-465.
    Shelton, D., Kim, Y., & Barak, G. (2006). A study of juror expectation and demands concerning scientific evidence: Does the CSI effect exist? Vanderbilt Journal of Entertainment and Technology Law, 9, 331-368.
    Smith, L.L., Bull, R., & Holliday, R. (in press). Understanding juror perceptions of forensic evidence: Investigating the impact of case context on perceptions of forensic evidence strength. Journal of Forensic Sciences.
    Thomas, A. (2006). The CSI effect: Fact or fiction. Yale Law Journal Pocket Part, 70, 70-72.