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Factors affecting jury decision-making:. U.K. legal system:. Magistrates’ court: 2 or 3 magistrates, or 1 district judge (no jury) Tries criminal “summary offences” (most motoring offences, drunk and disorderly). Passes “indictable offences” to Crown Court.

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U.K. legal system:

Magistrates’ court:

2 or 3 magistrates, or 1 district judge (no jury)

Tries criminal “summary offences” (most motoring offences, drunk and disorderly).

Passes “indictable offences” to Crown Court.

2011: 1.68 million proceedings, 166,808 trials.

County court:

No jury

Tries most civil offences

2011:1,553,983 (non-family) cases

Crown Court:

Judge and jury

Tries indictable offences (murder, rape, burglary)

plus appeals against magistrates’ court’s verdicts.

2011: 91,910 trials, 170,421 jurors

“Bench trial” Judge(s) only

(Permissible under Criminal Justice Act 2003 if there is a “real and present danger” of jury tampering).


Estimated number of U.S. jury trials per year:

(Juries used for both civil and criminal cases).

154,000 (149,000 in state courts, 5,000 in federal courts)

66% criminal trials (47% felonies, 19% misdemeanours)

31% civil trials

4% other

Source: State-of-the-States Survey of Jury Improvement Efforts (April 2007), National Center for State Courts.

Many cases settled before trial, by plea-bargaining.


Global variations in legal systems:

U.K./ U.S.-style “adversarial” systems:

Prosecution versus defence; jury assess guilt/innocence on basis of evidence; judge is a passive adjudicator, decides points of law and sentencing.

U.S.: variations between States, but generally 12 jurors, unanimous for “guilty/not guilty” decisions. Most cases settled by plea bargain, not trial.

U.S., Canada, Australia, N.Z: defendant can opt for trial by judge alone.

England, Wales: 12 jurors, unanimous vote, but 10:2 majority accepted if deliberations are protracted (over 2 hours, 10 minutes!).

Northern Ireland: Diplock courts (trial by judge alone) for terrorist offences.

Scotland: 15 jurors, simple majority.

Inquisitorial systems (e.g. France, Germany, Italy):

No jury except for major crimes; “lay assessors” (like magistrates) instead. Judge collects evidence (for/against defendant), calls witnesses, asks questions; primary role is active investigator.


Opinions on trial by jury:

Blom-Cooper, QC: "A judge can do the work of twelve amateurs, and better", Times (London), October 21st, 2003:

"The jury is the high point of amateurism, potentially a recipe for incompetence and bias. The mood of civilised systems of criminal justice increasingly demands professionalism. I am not contemptuous of the amateur’s ability to judge human conduct, only the task of evaluating evidence in the courtroom, which is a job for professionals".

Robert Frost (attrib.):

“A jury consists of twelve persons chosen to decide who has the better lawyer”.


Strengths and weaknesses of juries:

Problems with juries –

Untrained amateurs, varying in intelligence, education, etc.

Susceptible to media influence, emotionality of case, prejudice.

Secretive – jury decision-making process is not revealed.

Jurors may have difficulty with legal process –

confusion about “beyond reasonable doubt”, limiting instructions, joinder instructions, inadmissible evidence.

Jurors have difficulty with evaluating scientific and statistical evidence (the “CSI effect”).

Advantages –

A check on State power.

Juries are culturally diverse, unlike judges.


Assessing jury performance:

Bornstein and Greene (2011):

Usually impossible to know if jury verdict was correct, so can only assess performance in terms of:


Do jurors understand judge’s instructions to them?

Reliance on evidence:

Do jurors use the evidence they are supposed to use, and ignore extraneous information?

Comparison to expert decision-makers:

How do jurors’ decisions compare to those of judges?


Steblay et al (1999):

Meta-analysis on effects of pre-trial publicity..

44 studies (5,755 participants).

After exposure to negative PTP . participants were more likely to convict than participants exposed to positive or no PTP.

Effects stronger with real jurors and real PTP than with student mock-jurors, and with longer delays between PTP and judgement..

"PTP effects survive attempts at jury selection, continuance, judicial instruction, trial evidence and jury deliberation".

Change of venue is most effective remedy.

Consistent with "story model" of juror decision-making: biased schema affects subsequent interpretation of evidence.


Steblay et al (2006):

Jurors may be instructed to ignore inadmissible evidence or follow a “limiting instruction” (consider the evidence for one purpose but not another – e.g. to assess competence but not guilt).

Meta-analysis of 48 studies on ability of jurors to disregard inadmissible evidence.

Inadmissible evidence (unchallenged) affects verdicts in line with the evidence.

Judicial instructions to ignore it do not eliminate its effects.

Effectiveness of instructions to ignore evidence was enhanced if judge explained why the evidence was inadmissible.

Contested evidence that was subsequently ruled inadmissible accentuated the effects of that evidence.


Kalven and Zeisel (1966):

Study of 3,576 1950’s criminal trials in U.S. Jury verdict, plus judge questionnaire on details of case and judge’s own verdict.

Good agreement for convictions between juries and judges (78%).

Asymmetry in judge-jury disagreements: juries tended to acquit when judges would convict (19% of cases), much more than juries tended to convict when judges would acquit (3% of cases).

The judges' opinions were that most disagreements occurred due to –

different interpretations of “reasonable doubt” (more stringent for juries)

different evaluation of evidence

juror sentiments about the defendant

juror disagreement with the law

NOT due to jury misunderstanding of evidence


Eisenberg et al (2005):

Replicated Kalven and Zeisel with smaller sample, but more detailed analysis. Studied judges, jurors and attorneys.

Similar rates of judge-jury agreement on convictions (75%) as Kalven and Zeisel, despite social changes since 1950’s.

Similar asymmetry in judge-jury disagreements.

Judges would tend to convict more than juries overall, but this depended on strength of evidence.

Weak evidence – judges tend to acquit more than juries.

Both perceived evidential complexity and legal complexity were unrelated to judge-jury disagreements.

Male jurors were more willing to convict than judges.

More highly educated juries were less willing to convict.


The prosecutor’s fallacy:

Forensic experts are often asked to what extent a piece of evidence found at a crime scene can be linked to the suspect.

Keijser and Elffers (2012):

Suppose a suspect has the same blood group as a blood stain found at the crime scene.

Logically correct inference: a match is highly likely if the suspect produced the stain.

Logically incorrect inference: a match shows it is highly likely that the suspect produced the stain. (Incorrect because many of the population have that blood group).

Traditional way of expressing this information is the latter, but that is logically incorrect.

Schum (1994): need to distinguish between reliability of evidence and its diagnosticity.


Dead crewman - definitely Vulcan blood like Spock’s (highly reliable evidence)

Where did the murder take place?

Planet Vulcan – evidence is not diagnostic (Vulcan is full of green-blooded individuals)

Starship Enterprise – evidence is highly diagnostic (Spock is the only Vulcan on board)


Diagnosticity is given by a likelihood ratio.

= the likelihood of finding the evidence under the hypothesis that it came from a particular source (the suspect) divided by the likelihood of finding it under the hypothesis of an alternative source (someone from the general public).

e.g. if many people have that blood-group, the evidence is not diagnostic (it could be blood from many different people).

If only a handful of people have that blood-group, the evidence is more diagnostic, and the likelihood ratio is smaller.


Keijser and Elffers (2012):

compared3 groups on ability to interpret two fictitious forensic reports about evidence from a robbery.

Judges and lawyers had great difficulty understanding likelihood ratios; forensic experts were better but still poor.

Self-ratings of understanding were high for all groups (but unwarranted for judges and lawyers, who were near-chance).


Lieberman and Krauss (2009):

Verdict-driven juries –

Early vote and then discuss verdict options.

Evidence-driven juries –

Evaluate evidence and attempt to identify the truth. Late vote.

Verdict requirement affects how the jury behaves –

When unanimity is required, juries tend to be evidence-driven and more thorough.


How do jurors arrive at their decision?

Complex task: hear contradictory accounts of past events, complex forensic and statistical evidence.

Pennington and Hastie (1992):

“Story model” of decision-making process.

Pre-deliberation phase: jurors independently evaluate conflicting information and construct a story that provides a plausible explanation of evidence.

Filter information through own experiences, beliefs, etc.

Influence of schemas, cognitive heuristics, emotions.

Deliberation phase: jurors reconcile their differences (social psychology of group decision making).


Pennington and Hastie (1981):

Jurors tended to organise evidence into a story, often including evidence that was inferred and not actually presented during the trial.

Jurors who decided "guilty" or "not guilty" produced different stories.

Pennington and Hastie (1988):

Mock jurors recognised more evidence consistent with their verdict than inconsistent with it; also falsely recognised verdict-consistent evidence not actually presented.

When prosecution presented evidence in a story format and defence did not, jurors convicted defendant 78% of time. Opposite strategies: convicted 31% of time. The easier it is to construct a story, the more likely jurors produce a verdict consistent with that story.


Evaluation of expert evidence by jurors:

McCabe, Krauss and Lieberman (2010):

How likely is a “sexual violent predator” to reoffend?

In U.S., SVPs can be subject to “civil confinement” after prison.

Intuition-based clinical judgements are less accurate than “actuarial” estimates, but are more influential with mock-jurors.

Krauss, McCabe and Lieberman (2011):

Real-world sample of (unused) jurors.

Saw videotaped re-enactment of a real trial, with actor providing either actuarial or clinical expert testimony (identical conclusion – “likely to reoffend”).

Actuarial testimony: 15% increase in jurors’ “civil commitment” verdicts from pre- to post-trial.

Clinical testimony: 34% increase, plus more confident in their verdict.


Cognitive-Experiential Self-Theory (CEST):

Epstein (1994): two information-processing modes –

Experiential–”gut-level”, emotionally-based system. Processes information rapidly and effortlessly (default mode). Associative, analogical, relies on heuristics (Tversky and Kahneman, 1974, 1982).

Rational – analytical, intentional and effortful system, using logic and evidence.

Situational demands and individual differences affect which system predominates.

One of numerous "dual process" models of reasoning -

Chen and Chaiken (1999): "heuristic-systematic" model.

Kahneman (2011): “System 1” (fast, instinctive, emotional) and “System 2” (slow, deliberative, logical) thinking modes.


Effects of inducing rational or experiential bias by pre-trial instructions:

(e.g. Lieberman, 2002; Krauss, Lieberman and Olson 2004; Lieberman and Krauss 2009).

“When forming an opinion of the defendant, carefully consider all the evidence presented.Try to be as rational and analytic as possiible".


"When forming an opinion of the defendant, go with your natural, intuitive response. We are interested in your gut-level reactions to this case".

Processing mode affects influence of diagnostic labels (e.g. "psychopath"); use of actuarial versus clinical expert testimony; effects of emotional evidence; effects of defendant attractiveness


Gunnell and Ceci (2010):

Are experiential processors more influenced by extra-legal biases than rational processors?

Participants read a criminal trial transcript, accompanied by either an attractive or unattractive face.

E-processors and R-processors convicted attractive defendants at similar rates, but E-processors were more likely than R-processors to convict unattractive defendants.

E-processors gave harsher sentences to unattractive defendants and more lenient sentences to attractive ones (R-processors did not).



Jurors are not passive rational information-evaluators.

Influenced by numerous “extra-legal” factors.

Generally good agreement between juries and legal experts (who can also be prone to bias).

Need more information on how individual decisions are translated into group decisions, and more information on real-world jury behaviour.