Encyclopedia of Psychology and Law

evidence would match the defendant if he or she is

guilty, divided by the probability that the new evidence

would match an innocent person). The product is the

posterior odds. For example, after opening statements

and eyewitness testimony, a juror might believe that

there is a 25% chance that the defendant is guilty. The

prior odds are 25:(100−25) =.33:1. If the defendant and

the perpetrator share a blood type found in only 5% of

the population, the likelihood ratio is 1:.05 =20. The

posterior odds, then, are .33:1 × 20 =6.67:1. The prob-

ability of guilt is 6.67/(6.67 +1) or .87. In short, for this

juror, Bayes’s theorem states that the probability of

guilt should increase from .25 to .87.

Only a handful of studies have compared jurors’ deci-

sions with Bayesian norms, and the comparisons some-

times are difficult to make. Some studies have not asked

for a prior probability, while others have requested

beliefs that the evidence matched (instead of beliefs

about guilt). Most have assumed that jurors accepted the

statistical evidence at face value. Given these caveats, in

general, jurors underuse statistical evidence, compared

with a Bayesian norm. This general finding, however,

masks underlying complexity. In many of these studies,

the prior evidence, the statistical evidence, or both are

relatively strong. In such cases, it is difficult to exceed

Bayesian posterior probabilities (which are often .90 or

greater). Also, there tends to be great variability in how

jurors use the statistical evidence. That is, two jurors with

identical prior probabilities may hear the same statistical

evidence and arrive at very different posterior probabili-

ties. These disparities may rest in part on differing

expectancies about LEs (which typically have not been

presented) or about other factors (e.g., potential investi-

gator misconduct) affecting the value of the statistical

evidence. But studies (reviewed above) of how jurors

respond to statistical information by itself provide ample

reason to suspect wide variation in jurors’ understanding.

For example, jurors who claim to be comfortable with

mathematics are more likely to be affected by statistical

information than those who express discomfort. To fur-

ther complicate matters, at least one study has found that

later, nonprobabilistic evidence leads to a reevaluation of

the quantitative evidence presented earlier.

Does instruction help jurors combine the statistical

evidence with nonstatistical evidence? Studies have

provided simple instructions. Typically, they have

included a statistician’s testimony about how Bayes’s

theorem works. The expert displays a table or a graph

showing some sample prior probabilities and, given

the statistical evidence, corresponding posterior

probabilities. These relatively unsophisticated means of

instruction, generally, have not affected jurors’ use of

the evidence; jurors who receive the instruction come

no closer to Bayesian norms than those who do not.

Brian C. Smith

See alsoComplex Evidence in Litigation; Expert

Psychological Testimony; Jury Competence; Jury

Deliberation; Story Model for Juror Decision Making

Further Readings

Koehler, J. J., & Macchi, L. (2004). Thinking about low-

probability events: An exemplar-cuing theory.

Psychological Science, 15,540–546.

Levett, L. M., Danielsen, E. M., Kovera, M. B., & Cutler, B. L.

(2005). The psychology of jury and juror decision

making. In N. Brewer & K. D. Williams (Eds.),

Psychology and law: An empirical perspective

(pp. 365–406). New York: Guilford Press.

Niedermeier, K. E., Kerr, N. L., & Messe, L. A. (1999).

Jurors’ use of naked statistical evidence: Exploring bases

and implications of the Wells effect. Journal of

Personality and Social Psychology, 76,533–542.

Schklar, J., & Diamond, S. S. (1999). Juror reactions to DNA

evidence: Errors and expectancies. Law and Human

Behavior, 23,159–184.

“STEALINGTHUNDER”

In the context of the courtroom, “stealing thunder”

refers to revealing damaging information first so as to

diffuse its impact. If damaging evidence is going to be

brought out by one’s adversary, an attorney may choose

to reveal this information first to the judge and/or jury,

thereby stealing the adversary’s thunder. This entry

describes research on the effectiveness of stealing thun-

der and also explores why stealing thunder works.

Trial lawyers have concerned themselves with sev-

eral questions related to stealing thunder, and the

answers are usually a matter of conjecture based on

learned opinion and experience.

First, do attorneys use this tactic? The answer, based

on interviews, informal surveys, trial advocacy books,

and observations of courtroom trials, is that they do. In

fact, stealing thunder appears to be used unquestion-

ingly in criminal and civil trials, by defense, prosecu-

tion, and plaintiff’s attorneys.

764 ———“Stealing Thunder”

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