Jury Decision Making 1

Question 1

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

Answer 1

154,000 (149,000 in state courts, 5,000 in federal courts)
66% criminal trials (47% felonies, 19% misdemeanours)
31% civil trials
4% other

Question 2

Global variations in legal systems:

Answer 2

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.

Question 3

Strengths and weaknesses of juries:

Answer 3

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.

Question 4

Assessing jury performance: Bornstein and Greene (2011):

Answer 4

Comprehension, Reliance on evidence and comparison to expert decision makers.

Question 5

Daftary-Lapur, Dumas and Penrod (2010): Review of research on threats to rational decision-making by juries.

Answer 5

Jurors have poor comprehension of the standard legal instructions to them.

Consequences –

• Over-reliance on more confident (but not necessarily more knowledgeable) jurors during the deliberation process.
• Reliance on extra-legal evidence as opposed to trial evidence and testimony.
• Problems with concept of reasonable doubt, limiting instructions, joinder of charges, death penalty instructions.

Question 6

How well do jurors agree with judges?: Eisenberg et al (2005):

Answer 6

udges 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 judges and attorneys 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.

Question 7

Can jurors disregard inadmissible evidence? Effects of pre-trial publicity. Steblay et al 1999

Daftary-Kapur, Penrod, O’Connor and Wallace (2014):

Answer 7

44 studies (5,755 participants).
After exposure to negative PTP, participants were more likely to convict than participants exposed to positive PTP or no PTP.
Effects stronger with real jurors and real PTP than with student mock-jurors, and with longer delays between PTP and judgement.

“Shadow jury” methodology. Mock jurors recruited in two locations:
(a) Effects of “natural” PTP: mock jurors in New York (venue in which trial took place).
(b) Effects of experimentally-manipulated PTP: mock jurors in Boston (unaware of NY case).
Results similar for (a) and (b), implying experimental studies of PTP have good external validity.

Question 8

How well can jurors understand expert evidence?:

The prosecutor’s fallacy:

Answer 8

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.

Question 9

Diagnosticity is given by a likelihood ratio.

Answer 9

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 that it came from an alternative source (someone from the general public).

Keijser and Elffers (2012):
Compared 3 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.

Question 10

“CSI effect” (Lieberman et al 2008):

Skolnick and Shaw (2001) :

Answer 10

1) Jurors unduly influenced by forensic evidence (a problem because DNA and fingerprint evidence are not infallible).
2 )Mock jurors given strong/weak physical evidence (shoeprint match or knife match) and EWT (how well the witness could see the crime and defendant).
Jurors more influenced by physical evidence, even when EWT was strong and physical evidence was weak.

Question 11

The problem of forensic evidence:

Answer 11

Forensic evidence is not infallible.
Innocence Project: forensic evidence was present in over half of the exonerated cases.
Forensic DNA evidence error-rate may be 1 in 100 (Koehler et al
False positives due to:
Similarity of genetic profiles (random match probability).
Laboratory error due to cross-contamination, mis-labelling, misinterpretation (laboratory error rate).

Question 12

Ways of communicating DNA evidence:

Koehler (1996):

Answer 12

1) Presented evidence as likelihood ratios (100:1) or frequencies (1 in 100). Former led to mock jurors being more certain the suspect was the source of the DNA.