What makes expert witnesses credible with jurors? When they show
confidence, but not bravado, a new study suggests.
The study was headed by Robert Cramer, a doctoral student in psychology at
the University of Alabama. Results appeared in the March Journal of the
American Academy of Psychiatry and the Law.
Participating in the study were more than 300 university undergraduates
taking an introductory psychology course who played mock jurors. The subjects
were randomly divided into three groups. Subjects in each group were shown a
video portraying two mock expert witnesses testifying about their evaluation
of a convicted murderer during a postconviction sentencing phase. But the
script was slightly altered for each of the three groups so that the two
witnesses displayed low confidence, medium confidence, or high
For example, the witnesses in the low-confidence video had awkward posture,
showed signs of anxiety, used a quivering tone of voice, and often said"
you know" to seek assurance. Witnesses in the medium-confidence
video had good posture, showed comfort and poise, moderately paced their
speech, and often used the term "I am reasonably certain."
Witnesses in the high-confidence video leaned forward, spoke loudly and
rapidly, and often used the term "I am certain."
After the mock jurors had viewed the video version to which they had been
assigned, they rated the mock witnesses with the Witness Credibility Scale,
which contains 20 items concerning confidence, likability, trustworthiness,
and knowledge and gives an overall credibility score.
The low-confidence witnesses were rated as significantly less credible than
both the medium-confidence and high-confidence witnesses, but the
medium-confidence witnesses were rated as significantly more credible than the
Or as Cramer and his group commented in their paper, "Confidence does
not equal credibility, which means, pragmatically speaking, that the most
effective witness may not be the supremely confident witness."
This finding stands, interestingly, in contrast to that of another
study—that lawyers and judges prefer expert witnesses who display high
Cramer and his colleagues also wanted to find out whether the mock jurors'
personalities had influenced their credibility ratings of the mock expert
witnesses. To find out, they evaluated each mock juror on the personality
traits of neuroticism, extroversion, openness, agreeableness, and
conscientiousness, then looked to see whether there were any significant links
between the personality traits that the mock jurors possessed and their
credibility ratings of the mock expert witnesses.
A significant link was found only for extroversion: mock jurors who scored
high on extroversion had rated mock witnesses as significantly more credible
than had mock jurors who scored low on extroversion. This was the case
regardless of whether mock witnesses had portrayed low confidence, medium
confidence, or high confidence.
Why might extroverted jurors find witnesses more credible than introverted
jurors do? Extroverts are positive, friendly, gregarious individuals who seek
out positive interactions with other people. So extroverted jurors may find
expert witnesses credible simply because they, the jurors, feel a positive
rapport with them, Cramer and his group speculated. And if that is indeed the
case, they added, then extroverted jurors might find expert witnesses
especially credible if the witnesses themselves are extroverted.
"Our findings have practical implications for both witness
preparation and jury selection," the researchers concluded. For
instance, if expert witnesses have good posture, but do not lean forward to
persuade jurors; speak with a fair amount of certainty, but not cockiness; and
speak at a measured pace rather than rapidly, they are apt to exude just the
right amount of confidence to persuade jurors that their testimony is
accurate, the researchers believe. And if lawyers want jurors to believe
expert witnesses, then they might do well to select extroverted jurors.
The study was funded by the American Society of Trial Consultants.
"Expert Witness Confidence and Juror Personality: Their Impact
on Credibility and Persuasion in the Courtroom" is posted at<www.jaapl.org/cgi/reprint/37/1/63>.▪