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Expert Witness Confidence and
Juror Personality: Their Impact
on Credibility and Persuasion
in the Courtroom
Robert J. Cramer, MA, Stanley L. Brodsky, PhD, and Jamie DeCoster, PhD
The present study was conducted to investigate the relationship between both expert witness confidence and juror
personality with expert witness credibility, as well as expert witness credibility with juror sentencing outcome.
Participants were presented with one of three randomly assigned filmed scenarios depicting various levels of
manipulated witness confidence. They then completed a sentencing outcome item, the Witness Credibility Scale,
and the Goldberg Five-Factor Markers. Expert witness confidence had a significant main effect on ratings of
credibility, with moderate levels of manipulated confidence yielding the highest credibility. Juror extroversion was
positively related to perceptions of expert witness credibility. Finally, juror ratings of expert witness credibility, as
well as two subcomponents, predicted juror sentencing outcome.
J Am Acad Psychiatry Law 37:63–74, 2009
Court testimony may cause many experts to be ap-
prehensive for a variety of reasons. For instance, law-
yers may pressure expert witnesses through intrusive
or aggressive questioning in cross-examinations,
1,2
and defendants occasionally threaten vindictive liti-
gation.
3
Such prior experiences may diminish expert
witness confidence and, as a consequence, negatively
affect their credibility. One method of bolstering ex-
pert confidence is witness preparation. Boccaccini
and Brodsky
4
noted that witness preparation in-
structs the witness on how to communicate honestly
and effectively to inform and persuade jury mem-
bers, as well as to boost witness confidence. Witness
confidence by itself has been hypothesized to increase
perceived witness accuracy and believability.
5
In the present study, we examined behaviors re-
flecting expert witness confidence as predictors of
perceived expert witness credibility. Juror individual
difference factors may alter perceptions of expert wit-
ness credibility. Thus, we also assessed the associa-
tion between juror personality and perceptions of
expert credibility. The expert testimony literature
will benefit from the present study in several ways.
First, examining constellations of witness behaviors
connoting confidence will add to the existing knowl-
edge base for verbal and nonverbal targets of witness
preparation. Second, extant work in the testimony
area lacks assessment of the impact of a comprehen-
sive framework of juror personality traits. Accom-
plishing such an analysis will facilitate understanding
of how person-related characteristics influence per-
ceptions of credibility.
We begin with a review of the literature on defin-
ing expert witness credibility. Then, we focus on de-
fining confidence and associated behaviors in the
context of expert testimony. We next establish a basis
to investigate the effect of juror personality on per-
ceptions of credibility. Finally, we look at studies that
link credibility and jury decision-making to provide
a rationale for testing the impact of expert credibility
on sentencing outcome.
Expert Witness Credibility
A decision to utilize expert testimony includes a
consideration of the likely impact of that testimony
on the trial process and verdict.
6
Research on expert
Mr. Cramer is a doctoral student, Dr. Brodsky is Professor of Psychol-
ogy, Dr. Decoster is Director, Social Cognition Laboratory, The Uni-
versity of Alabama, Tuscaloosa, AL. This paper was presented at the
annual meeting of the American Psychology-Law Society in St. Peters-
burg, Florida, March 2006. The project was supported by a grant from
the American Society of Trial Consultants. Address correspondence to:
Robert J. Cramer, MA, University of Alabama, Box 870348, Tusca-
loosa, AL 35487. E-mail: crame001@bama.ua.edu
63Volume 37, Number 1, 2009
REGULAR ARTICLE
credibility in the courtroom is one means of investi-
gating the impact of testimony.
7
Credibility is a mul-
tifaceted construct comprising numerous compo-
nents including, but not limited to, believability,
8,9
credentials,
10
and likability.
8
Williams and Mc-
Shane
7
succinctly defined credibility as the degree of
trust potential jurors ascribe to the expert. In a
broader scope, Boccaccini and Brodsky
4
assessed the
role of locality of practice, work experience, testifying
experience, and fee for testifying in the believability
of expert witnesses in a survey of 488 adults. Expert
witnesses were rated as most believable if they were
from the same community as the participants, pro-
vided psychotherapy to clients, had previously testi-
fied for both the prosecution and the defense, and
were not paid for their testimony. Gutheil
11
sug-
gested that other components of effective or credible
expert testimony include use of visual presentations
(i.e., charts), adjustment of language to the level of
understanding of the jury, use of an appropriate nar-
rative style, and avoidance of direct criticism of op-
posing expert witness testimony.
Research on source credibility has taken place
apart from the field of witness testimony. Mc-
Croskey
12
developed an early measure of source credi-
bility that was subsequently revised
13
to include
components such as good will, competence, and
trustworthiness. Research utilizing this measure in
business- and relationship-related work has shown
credibility to be inversely related to verbal aggres-
siveness and communication apprehension.
14
Other
early conceptualizations of credibility included
components such as competence, dynamism, and
objectivity.
15,16
Studies of source credibility (outside the court-
room) indicated that persons who employ slow rates
of speech are viewed as more calm, composed, trust-
worthy, and honest by evaluators in comparison to
those who utilize rapidly paced speech.
17
Speak-
ers who used rapid speech were seen as more dynamic
and extroverted, but these qualities did not reflect a
high degree of credibility. Overall, our definition of
witness credibility drew on theoretical
12,13
and re-
search perspectives on source credibility.
8–10
We
now turn to the nature of confidence.
Confidence: Definition and
Associated Characteristics
Expert witness confidence has often been noted to
influence jurors.
18,19
It is important to distinguish
low confidence from anxious behaviors, however, to
avoid confounding interpretations of expert witness
performance. We define confidence as the degree of
demonstrable self-assurance expert witnesses have in
their general ability on the stand. This conception is
similar to the definition given by Slovenko
19
of wit-
ness confidence with the additional element of be-
havioral specification of levels of confidence.
Our working assumption is that expert witnesses
with low confidence exhibit verbal and nonverbal
cues characteristic of nervousness. While low confi-
dence and high anxiety may evoke similar behaviors,
we distinguish the two by asserting that anxiety or
nervousness is one emotional result of having low
confidence in one’s ability to testify. In other words,
the belief (confidence) precedes both the emotion
(anxiety) and behavior (cues of nervousness). Exam-
ples of such cues include a quivering voice and fixed
eye contact. While expert witnesses with medium or
high confidence in their findings may be anxious
about testifying, it is proposed that such witnesses
may exude few external cues of anxiety or nervous-
ness due to their assurance in their professional or
scientific abilities. In sum, anxious verbal and non-
verbal cues are seen as symptomatic only of low-
confidence expert witnesses.
Speech pattern may affect the perception of con-
fidence. For example, O’Barr
20
distinguished power-
ful from powerless speech as they relate to confi-
dence; powerful speech is seen as a function of social
status and reflects high confidence, while powerless
speech conveys lower social status and low confi-
dence. O’Barr found that mock jurors rated both
male and female witnesses who used powerful speech
as more convincing, truthful, competent, intelligent,
and trustworthy than their powerless counterparts.
Another differentiation in speech patterns on the
stand can be seen in formal speech versus hypercor-
rect speech. According to O’Barr, formal speech is
analogous to styles reflected in medium confidence
levels and includes usage of lay terminology, people’s
names, and easily understandable vocabulary. Hy-
percorrect speech styles, on the other hand, refer to
people in impersonal ways (i.e., “the client”), use
technical terminology, and pedantic word choice.
Mock-jurors rated expert witnesses as more convinc-
ing, competent, qualified, and intelligent when they
used a formal speech style in comparison to a hyper-
correct style.
20
Drawing on these findings, we uti-
lized characteristics of the hypercorrect style in the
Expert Witness Confidence
64 The Journal of the American Academy of Psychiatry and the Law
high-confidence condition, because impersonal lan-
guage may be interpreted as highly confident or ar-
rogant, whereas formal speech facets were used to
reflect medium confidence.
Thomas and McFayden
21
stated that people ex-
press levels of confidence equal to the degree of as-
surance they have in their knowledge or beliefs. Ac-
cording to this perspective, highly confident expert
witnesses may indicate that they are “absolutely cer-
tain” about their findings and act accordingly,
whereas a moderately confident expert who is “rea-
sonably certain” may give fewer cues of confidence.
There are mixed results from studies and observers of
the effects of expert witness confidence. A prelimi-
nary study found that moderate levels of confidence
have a greater effect on mock jurors’ decision-making
processes than do higher or lower levels of confi-
dence.
22
In contrast, judges and lawyers have been
reported to prefer experts who are highly confident
and make definitive conclusions.
23
From these stud-
ies, we can see that speech patterns and content of
speech are associated with confidence.
Results on the association between confidence and
credibility have been equivocal concerning what de-
gree of confidence is most credible. The Confidence
Heuristic Model
24–27
explains the relationship be-
tween confidence and perceiver judgment. This per-
spective holds that receivers of information make
quick, surface judgments when presented with a large
amount of information. In the case of judgments of
expert or eye witness confidence, receivers judge the
most confident communicators as being most accu-
rate and/or credible.
28
In relation to self-rated judg-
ments of confidence, Pulford and Colman
25
re-
ported findings supporting the confidence heuristic
in a sample of 56 participants who had to identify
potential perpetrators of crimes. Participants with
higher self-reported confidence in their identifica-
tion of a suspect as perpetrator consistently per-
suaded their partners to arrest their suspects. The
confidence heuristic model may indeed lend support
to rating high expert confidence as the most credible.
Empirical studies show a strong relationship be-
tween confidence and persuasion.
21,29
For in-
stance, an early study that tested such a hypothesis
yielded a curvilinear relationship between experi-
mental conditions of low, moderate, and high confi-
dence with persuasion,
30
such that the greatest per-
suasion was found for messengers with a moderate
level of confidence. These studies provide a basis to
hypothesize that juror perceptions of credibility will
show a curvilinear relationship as a function of wit-
ness confidence, with the highest ratings of credibil-
ity being associated with medium expert confidence.
Given the contradictory evidence between confi-
dence-persuasion literature and the confidence-heu-
ristic work, we sought in the present study to inves-
tigate which perspective was supported in the context
of expert witness confidence and credibility.
The present study features two confidence con-
structs. The first, referred to from here on as manip-
ulated confidence, comprises the behaviorally ma-
nipulated levels of confidence based on this section of
the literature review. The second, referred to as per-
ceived confidence, consists of the subscale from the
Witness Credibility Scale on which mock jurors rate
the expert. We make this distinction for the sake of
conceptual clarification. Also, statistical analyses in-
volving both manipulated confidence and total cred-
ibility were run with and without the perceived-
confidence subscale, to avoid conceptual confounds.
Juror Personality Traits as Predictors
of Decision-making
Juror personality traits may influence their evalu-
ation of evidence. Previous research on juror decision-
making has particularly focused on juror demo-
graphic variables such as sex, education, and
occupation, in combination with traits of the expert
such as credentials, physical appearance, and com-
municative ability.
10
Other factors that influence ju-
ror perceptions of evidence include age of the wit-
ness
31
and prior jury experience.
32
While demographic and experiential factors have
been investigated, in-depth constellations of person-
ality traits have largely been ignored in the literature.
The five-factor model (FFM)
33
is a commonly ac-
cepted conception of personality structure. The five
factors are neuroticism, extroversion, openness to ex-
perience, agreeableness, and conscientiousness.
34
Each of the five domains comprises six subcom-
ponents, or facets, that make up the respective
unique qualities. General descriptions of each do-
main, including several facets from each domain, are
as follows
33
:
Neuroticism involves the degree of emotional
stability in forms such as impulsivity, depression,
anger, and anxiety.
Cramer, Brodsky, and DeCoster
65Volume 37, Number 1, 2009
Extroversion assesses the degree to which a per-
son seeks external stimulation and comprises fac-
ets such as positive emotions, excitement seek-
ing, and gregariousness.
Openness to experience examines imagination or
intellectual acumen, and includes openness to
ideas, fantasy, and esthetics.
Agreeableness describes a person’s interactional
disposition in ways such as level of altruism,
warmth, and sympathy.
Conscientiousness incorporates self-control in
forms including being organized, determined,
and disciplined.
Research applications of the FFM have included
investigations of personality stability, relationship
to affective states (e.g., depression, anxiety), and
delineation of other personality factors (e.g.,
spirituality).
33,35,36
Juror personality traits may influence perceptions,
and there are a few investigations of five-factor traits
in relation to juror belief patterns.
37–39
However,
juror personality research has focused predominantly
on constructs such as dogmatism and authoritarian-
ism,
40
both of which emphasize closed-minded, rigid
thinking (directly opposed to the FFM factor of
openness to experience). Narby et al.
41
conducted a
meta-analysis of studies that measured juror verdict
preferences and two forms of authoritarianism: tra-
ditional and legal. Both forms of authoritarianism
were consistently associated with juror verdicts across
20 studies.
Research on the five-factor traits has shown an
association between extroversion and alteration of
decisions after group deliberation, favoring the plain-
tiff in civil trial outcomes.
38,42–44
This body of liter-
ature also suggests jurors high in openness are less
influenced by fellow jurors, and those high in con-
scientiousness are more influenced by their peers.
42
One study that examined the FFM domains and
credibility focused solely on openness. This study
found that openness was positively associated with
persuasion in a sample of 200 university students in
India.
45
In other words, students higher in openness
to ideas were more likely to be persuaded by infor-
mation presented to them. This finding contradicts
how general openness relates to persuasion from
other jurors, but is possibly accounted for because of
the narrow definition of openness used in the study.
Expert Witness Testimony and
Jury Sentencing
The sentencing phase of capital murder cases, a
context in which expert witnesses commonly testify,
has drawn interest to the link between expert witness
testimony and juror sentencing recommendations.
46
Jurors often take into account the opinion of an ex-
pert mental health witness on factors such as the
causes of violence by the defendant and the likeli-
hood of future violence.
47
Williams and McShane
7
reported that credibility of psychological testimony
has a significant effect on death sentence recommen-
dations. In this case, expert testimony about the de-
fendant’s insanity was considered by potential jurors,
and, overall, the likelihood of death sentences de-
creased compared with prior ratings.
The Present Study
The available literature lacks sufficient informa-
tion on expert witness confidence and associated be-
haviors as they relate to perceptions of credibility.
Moreover, this investigation offers one of the first
looks at juror personality as it relates to expert testi-
mony. With these goals in mind, the hypotheses were:
H1: Highest ratings of expert witness credibility
are expected in the medium manipulated confi-
dence condition.
H2: Juror personality accounts for significant
variability in witness credibility.
H3: Expert witness credibility is positively re-
lated to congruent juror sentencing decisions;
that is, higher ratings of expert credibility are asso-
ciated with greater agreement by mock jurors.
Method
Manipulation and Pilot Study
A script of courtroom sentencing testimony orig-
inally developed by Krauss and Sales
48
to study actu-
arial versus clinical testimony was adapted for the
current study. Additional research employing these
scripts has been conducted by Krauss and Lee.
49
The
script portrays forensic expert witnesses testifying
about their evaluation of a convicted murderer dur-
ing the sentencing phase, describing the high likeli-
hood that the defendant will commit future violent
acts. The video presents a prosecution expert testify-
ing that a defendant convicted of murder is likely to
commit future violent acts.
Expert Witness Confidence
66 The Journal of the American Academy of Psychiatry and the Law
Three versions of the script were used to portray
the three levels of manipulated confidence. Two
male witnesses were videotaped at the three different
levels of confidence. The actors were extensively
trained using repeated practice, video tape review,
and pilot studies to ensure consistency and accuracy
of portrayed levels of confidence.
As a manipulation check, a confidence item on a
10-point Likert scale was analyzed to ensure that the
varying manipulation levels conveyed the desired rat-
ings of levels of confidence. It read: “How confident
would you rate the witness?” Participants rated on a
scale of 1 (low confidence) to 10 (very high confi-
dence). Also, an open-ended question asking partic-
ipants about aspects of the testimony on which they
based their determination of perceived confidence, as
well as an adjective checklist adapted from the Eval-
uation of Others Questionnaire,
50
was included to
detect the presence of other constructs. The manip-
ulation was successful in that desired differential rat-
ings of confidence were obtained. Also, participants’
perceptions of what the study was about were con-
cluded not to have influenced their judgments of the
expert witnesses.
A pilot study was conducted to verify that the
different scenarios represented different levels of
manipulated confidence. A 2 ⫻3 multifactor
analysis of variance (ANOVA) was conducted to
test the effect of actor and manipulated confidence
level on the mock juror’s ratings of perceived con-
fidence. There was a significant main effect of ma-
nipulated confidence level on the rating of per-
ceived confidence (F(2,79) ⫽98.31, p⬍.001).
Least significant difference (LSD) post hoc com-
parisons revealed significant mean differences be-
tween each level of manipulated confidence with
every other level (p⬍.001 for all comparisons).
Based on these results, we concluded that the ma-
nipulated levels of confidence accurately reflected
differing levels of perceived confidence. The inter-
action between actor and manipulated confidence
level was not significant (F(2,79) ⫽1.63, p⫽
.205). This finding indicates that the particular
actor observed made no difference in mock juror
ratings of perceived confidence at each level of the
manipulation.
The study was approved by the University of Ala-
bama Institutional Review Board for the Protection
of Human Subjects.
Materials
Demographics
Participants completed a demographic form in-
quiring about their age, sex, ethnicity, religious ori-
entation, major/intended major in school, attitudes
toward the death penalty, and previous experience
serving on a jury. Religious orientation included op-
tions to identify oneself as Catholic, Protestant,
Christian (other), Jewish, Agnostic, Atheist, or
Other. Christian (other) was added to distinguish the
variety of southern Christian denominations (e.g.,
Southern Baptist) from other predominant catego-
ries such as Catholicism and Protestantism.
Manipulated Expert Witness Confidence
Levels of manipulated confidence were portrayed
through scripted videotapes. The altered verbal and
nonverbal components of each level of confidence
are described below. Three levels were assessed: low,
medium, and high confidence.
Low confidence: quivering tone of voice, dysflu-
encies in speech, vacillating pace of speech, cor-
rections, breaks in the flow of words, postural
awkwardness, fixed eye contact, saying “you
know” to seek assurance, asking for repetition of
questions, and signs of anxiety and nervousness.
Medium confidence: moderate and stable tone of
voice, clarity in speech, moderately paced speech,
willingness to acknowledge a degree of certainty
(“I am reasonably certain”), smooth narrative
statements, good posture and straight back, com-
fort and poise, consistent eye contact, accurate
hearing and appropriate responses.
High confidence: loud and strong tone of voice,
assertive speech and mannerisms, rapidly paced
speech, always and all statements (“I am cer-
tain”), good posture/leaning forward, high flu-
ency of speech.
Personality Traits
Personality traits were measured with the Gold-
berg Five-Factor Markers (GFFM).
51,52
This brief
measure is based on a personality inventory NEO-
PI-R
34
and measures five personality domains: neu-
roticism/emotional stability (N), extroversion (E),
openness to experience (O), agreeableness (A), and
conscientiousness (C). The GFFM consists of 50
questions scored on a 1–5 Likert scale. Cronbach
␣
reliabilities have been reported as follows: .86 for N,
Cramer, Brodsky, and DeCoster
67Volume 37, Number 1, 2009
.87 for E, .84 for O, .82 for A, and .79 for C.
51,52
Internal consistencies for the present investigation
were similar: .77 for N, .87 for E, .74 for O, .78 for A,
and .79 for C.
Expert Witness Credibility
Expert credibility was assessed with the Witness
Credibility Scale.
8
The scale contains 20 items, each
rated on a 10-point Likert scale. Prior factor analyses
yielded four separate, robust domains: confidence,
likability, trustworthiness, and knowledge.
8
The
␣
coefficients have been reported for each subscale and
are as follows: confidence (.88), likability (.86), trust-
worthiness (.93), and knowledge (.86). The subscale
internal consistencies in this investigation were .96
for confidence, .94 for likability, .96 for trustworthi-
ness, and .96 for knowledge. The four subscales are
totaled for an overall credibility score. Griffin et al.
8
reported
␣
⫽.95 for the total score. For overall cred-
ibility in the present investigation,
␣
⫽.97. For the
purposes of the present study, expert witness credi-
bility was defined by all four factors of the scale, as
well as the overall total credibility score.
Juror Sentencing
Juror recommendation of the death penalty was
assessed with a 10-point Likert-scale. The question
read: “Based upon the expert witness’s testimony,
how likely are you to recommend a death sentence?”
Procedure
Participants watched a randomly assigned condi-
tion of expert witness testimony and then completed
the witness credibility scale,
8
sentencing recommen-
dation item, and GFFM.
51,52
Data Analysis
All assumptions of equality of variances, normal-
ity, and independence were satisfied with one minor
exception addressed in the analysis discussion. In
analyses in which control variables were used, the
control set included: the juror’s sex, jury duty expe-
rience, ethnicity, and support for the death penalty.
Furthermore, analyses involving both expert confi-
dence and credibility were conducted once with and
once without the confidence items of the witness
credibility scale. In this way, the potential confound
of overlapping of manipulated expert witness confi-
dence and that subscale of the credibility scale was
addressed.
Results
Participants
Participants consisted of 317 undergraduates from
introductory psychology courses at a large university
in the southeastern United States. They were offered
research credit in their courses for participation. Of
the 317 participants, those satisfying the death qual-
ification criteria established in Witherspoon v. Illi-
nois
53
were eliminated from the analyses, to reflect an
externally valid jury pool. In other words, mock ju-
rors who were completely against assigning the death
penalty were eliminated from the data analyses. De-
mographic data for those participants are reported
below.
Of the original pool of 317 participants, 299 were
death-eligible. Of those, 96 were male, 201 were fe-
male, and 2 did not identify their sex. The mean age
was 18.97 years (SD ⫽2.64), with a range of 17 to
52 years. There were 94 participants in the low-
confidence group, 101 in the medium-confidence
group, and 104 in the high-confidence group. Par-
ticipants reported their ethnicity as follows: Cauca-
sian, 249 (83%); African-American, 36 (12%); His-
panic, 9 (3%); Asian-American, 2; other, 2; and 1,
unspecified. Participants reported their religious af-
filiation as follows: Catholic, 39; Protestant, 55;
Christian (other), 172; Jewish, 4; Agnostic, 10; Athe-
ist, 7; Other, 9; and unidentified, 3. Eight partici-
pants reported having jury duty experience; they
were retained in the analyses.
Witness Confidence Analyses
The graph in Figure 1 depicts the relationship be-
tween manipulated expert witness confidence and
total expert witness credibility. The one in Figure 2
displays the relationship between manipulated expert
witness confidence and expert credibility subscales.
ANOVA results showed a significant main effect
of manipulated expert witness confidence on per-
ceived total credibility (F(2,296) ⫽163.15, p⬍
.001), with significant differences in perceived cred-
ibility between pairs of all three manipulated-confi-
dence conditions. LSD post hoc analyses showed that
the low-confidence condition was rated as signifi-
cantly less credible than both the medium- and high-
confidence conditions (both p⬍.001), and the me-
dium-confidence condition was rated as significantly
more credible than the high-confidence condition
(p⫽.013). The same pattern was also found when
Expert Witness Confidence
68 The Journal of the American Academy of Psychiatry and the Law
the perceived-confidence subscale was removed from
the measure of credibility.
Analyses yielded a significant main effect of ma-
nipulated expert witness confidence on perceived
confidence (F(2,296) ⫽463.50, p⬍.001). Al-
though differences were significant between the
low-confidence witness and both the medium and
high-confidence witnesses (p⬍.001 for both com-
parisons), the difference between the medium- and
high-confidence witness on perceived confidence
was not significant (p⫽.297).
Similar significant main effects of manipulated ex-
pert witness confidence were found on perceived lik-
ability (F(2,296) ⫽14.05, p⬍.001), on perceived
trustworthiness (F(2,296) ⫽74.10, p⬍.001), and
on perceived knowledge (F(2,296) ⫽149.03, p⬍
.001). LSD post hoc tests revealed significant differ-
ences between the manipulated medium-confidence
witness with both the low- and high-confidence wit-
nesses on likability (p⬍.001 for both comparisons).
LSD post hoc tests revealed significant differences in
perceived trustworthiness and knowledge between
the low-confidence witness and both the medium-
and high-confidence witnesses (p⬍.001 for all com-
parisons). There was also a significant difference in
perceived trustworthiness between the medium- and
high-confidence witnesses (p⫽.003).
Juror Personality Analyses
Set regressions were used to test the ability of the
set of personality factors to predict subscales of per-
ceived witness credibility, as well as total credibility
with and without the confidence subscale. Cohen
and colleagues stated that set regression is warranted
when “there is some theoretical role shared by the
variables in the set” (Ref. 54, p 169). Therefore, as-
sessing personality as a set is justified because juror’s
individual personality traits do not exist apart from
each other. However, they also noted that examina-
tion of individual tests of factors in a set is still war-
ranted. Thus, each personality domain was also ex-
amined independently for predictive ability.
The personality set included measures of neuroti-
cism, extroversion, openness, agreeableness, and
Figure 1. Manipulated confidence by perceived credibility.
Cramer, Brodsky, and DeCoster
69Volume 37, Number 1, 2009
conscientiousness. These results are presented in Ta-
ble 1. The personality set column represents the
unique ability of the collection of personality vari-
ables to predict variance in each outcome above and
beyond control variables.
Results for personality showed significant trends
for both overall credibility (p⫽.10) and credibility
without the perceived-confidence subscale (p⫽.07).
These relationships were clarified by the fact that the
personality set was significantly related to witness
trustworthiness in particular (p⫽.02). Within the
personality domains, only extroversion was a signif-
icant predictor of components of credibility. Extro-
version was also associated with overall credibility
and credibility without the perceived-confidence
subscale.
Witness Credibility and Juror
Sentencing Analyses
A set regression was used to assess the ability of
witness credibility subscales (confidence, likability,
trustworthiness, and knowledge) above and beyond
control variables to predict variance in likelihood to
assign the death penalty. The effect of the set of con-
trol variables on likelihood to assign the death pen-
alty was significant (F(8,287) ⫽3.73, p⬍.001;
R
2
⫽1.09). Specifically, there was a significant pos-
itive main effect of juror support for the death pen-
alty on likelihood of assigning the death penalty
(t(303) ⫽4.84, p⬍.001). No other control vari-
ables were significant.
The effect of the set of witness credibility subscales
above and beyond control variables on assigning the
death penalty was significant (F(4,283) ⫽77.34, p⬍
.001; R
2
change ⫽.47). No effect of perceived wit-
ness confidence (t(283) ⫽.41, p⫽.68) or perceived
witness likability (t(283) ⫽⫺1.12, p⫽.23)
emerged. However, perceived witness trustworthi-
ness (t(283) ⫽3.63, p⬍.001) and perceived witness
knowledge (t(283) ⫽4.84, p⬍.001) were signifi-
Figure 2. Manipulated confidence by perceived credibility subscales.
Table 1 Juror Personality and Expert Witness Credibility Analyses
Dependent Variable
Personality Set
(F(5,282) ⫽)
Significant Personality
Domains t(282) ⫽)
Confidence 1.28 (p⫽.27)
(R
2
change ⫽.02)
None
Likability 1.01 (p⫽.41)
(R
2
change ⫽.02)
Extroversion
(2.06, p⫽.04)
Trustworthiness 2.65 (p⫽.02)
(R
2
change ⫽.04)*
Extroversion
(3.21, p⫽.001)
Knowledge 1.44 (p⫽.21)
(R
2
change ⫽.03)
Extroversion
(2.45, p⫽.02)
Total credibility 1.85 (p⫽.10)
(R
2
change ⫽.03)†
Extroversion
(2.67, p⫽.008)
Total credibility
w/o confidence
2.05 (p⫽.07)
(R
2
change ⫽.04)†
Extroversion
(3.00, p⫽.003)
The set of control variables was not significant in all analyses. All
effect size values represent the impact of set two (personality
variables).
*Significant set.
†Significant trend.
Expert Witness Confidence
70 The Journal of the American Academy of Psychiatry and the Law
cant positive main effects on the likelihood of assign-
ing the death penalty.
A multiple regression analysis was conducted to
assess the ability of control variables and total credi-
bility to predict the variability in likelihood of assign-
ing the death penalty. The overall model predicted a
significant amount of variance in likelihood of as-
signing the death penalty (F(9,286) ⫽35.72, p⬍
.001l R
2
⫽.53). Support for the death penalty was a
significant positive main effect on likelihood of as-
signing the death penalty (t(286) ⫽7.32, p⬍.001).
No other control variable was significant. A signifi-
cant positive main effect was found for total per-
ceived witness credibility (t(286) ⫽16.25, p⬍
.001).
We repeated the above analysis after removing the
confidence subscale from the total credibility mea-
sure. The overall model predicted a significant
amount of variance in likelihood of assigning the
death penalty (F(9,286) ⫽32.64, p⬍.001; R
2
⫽
.51). Again, both support for the death penalty
(t(286) ⫽6.78, p⬍.001) and total perceived witness
credibility (t(286) ⫽15.64, p⬍.001) were signifi-
cant positive main effects on recommending the
death penalty.
Discussion
Manipulated expert witness confidence showed a
curvilinear association, and juror extroversion
showed a positive association, with judgments of wit-
ness credibility. The influence of behaviors associ-
ated with expert confidence on receivers of informa-
tion supports prior research and commentary.
19,21,29
Our findings add that credibility was lower for high-
confidence expert witnesses than for their medium-
confident counterparts, while credibility increased
from the low- to the medium-confidence condition.
Why the curvilinear relationship between manip-
ulated confidence and credibility? For a start, manip-
ulated confidence was defined by verbal and nonver-
bal behaviors during testimony. During pilot
analyses, some mock jurors indicated that they used
posture, certainty of conclusions, and rate and flow
of speech to define their perceptions of expert confi-
dence. At higher levels of manipulated confidence,
mock jurors may have interpreted rapid rates of
speech as overly intense, unwillingness to admit un-
certainty in conclusions as cocky, and forward-lean-
ing posture as an excessive attempt to be persuasive.
Aspects of credibility such as trustworthiness and be-
lievability suffered in high-confidence witnesses
when compared with medium-confidence witnesses
who admitted a degree of uncertainty and spoke at a
moderate pace.
Our findings partially contradict those related to a
confidence-heuristic model.
25,28
This theory posits
that receivers of information judge credibility or ac-
curacy based on the messenger’s level of confidence,
instead of processing a significant amount of infor-
mation themselves. In other words, that there is a
positive linear association between perceptions of
confidence and credibility or accuracy. Although
mock jurors in our study rated medium-confidence
witnesses higher in perceived credibility than low-
confidence witnesses, this increasing pattern did not
hold true for the medium- compared with high-
confidence witnesses in the present study. The dif-
ferences between our findings in confidence-heuris-
tic work may be due to our participants’ making an
intentional correction in the high-confidence condi-
tion. If the expert was seen as unwarrantedly confi-
dent, the participants may have lowered their ratings
of credibility to compensate.
Personality as a whole seems only marginally re-
lated to perceptions of expert testimony as indicated
by significant trends between juror personality and
ratings of overall expert credibility. However, extro-
version appeared to affect juror perceptions of expert
witness testimony; those high on extroversion per-
ceived expert witnesses as more credible. Why was
extroversion the only five-factor trait that exerted sig-
nificant influence on perceptions of expert witness
credibility? Krishnamurthy
45
found openness to be
positively related to being persuaded, but utilized a
culturally diverse sample and a definition of openness
limited to openness to modern ideas.
While some researchers have found juror extrover-
sion to be important in group deliberations,
42,43
it
has not been examined in credibility research. Costa
and McCrae
34
characterized extroverts as gregarious
and outgoing persons who seek out positive interac-
tion with their environment. Our positive relation-
ship between juror extroversion and judgments of
expert witness credibility may be due to extroverts’
seeking out positive emotions. If this is the case, ex-
troverts may rate expert witnesses who are positive,
friendly, and talkative more favorably because they
provide positive emotions. Thus, extroverts identify
expert witnesses as trustworthy, likable, knowledge-
Cramer, Brodsky, and DeCoster
71Volume 37, Number 1, 2009
able, and therefore credible, according to the theo-
retical definition of credibility.
The present results replicated one finding of Wil-
liams and McShane,
7
in the sense that overall expert
witness trustworthiness was positively related to in-
fluence on juror sentencing recommendation. The
present study extends the current literature based on
an expanded definition of witness credibility to in-
clude trustworthiness, knowledge, confidence, and
likability. Williams and McShane limited their defi-
nition of credibility to juror trust of a witness. Com-
ponents of the broadly defined construct of perceived
credibility—namely, trustworthiness and knowl-
edge—impact life-and-death decision-making. In
this scenario, it appears that jurors who find an expert
both knowledgeable and trustworthy can comfort-
ably assign the death penalty to a criminal defendant.
It is plausible that these facets of credibility, as op-
posed to likability and confidence, alleviate the hes-
itation or uncertainty jurors have in agreeing with, or
following the opinion of, the expert witness. Al-
though Federal Rules of Evidence dictate that the
expert refrains from giving an opinion on the ulti-
mate legal issue (in this case sentencing outcome),
the expert provided evidence that the defendant was
a continuing danger to society and likely to recidi-
vate. Jurors may have an internal conflict about as-
signing the death penalty because of this uncertainty
about future dangerousness. However, they may be
more willing to recommend the death penalty in in-
stances in which the expert is perceived to possess
sufficient knowledge and to convey an air of trust or
integrity.
Implications of Present Findings
The present study necessitates a word about the
constructs of confidence and credibility in the con-
text of expert testimony. The significant associations
and lack thereof in some instances clarify how these
constructs function in expert testimony and jury de-
cision-making. Expert confidence does not have a
linear impact on credibility and decision-making. In
other words, confidence does not equal credibility,
which means, pragmatically speaking, that the most
effective witness may not be the supremely confident
witness. Also, we have identified a broad scope of
behaviors associated with confidence. This finding,
however, does not mean that the construct was com-
pletely assessed, nor does it illuminate which (if any)
facets of behaviorally defined expert confidence are
the most influential. Finally, although expert credi-
bility has shown four orthogonal factors in previous
studies,
8
it appears that different portions of the
construct of expert testimony actually relate to the
sentencing recommendation. The validity of the
four-factor definition of expert witness credibility,
therefore, is somewhat limited if we consider all four
equally important. Expert credibility, as theoretically
and empirically defined by the present study, may in
fact vary depending on the exact stage of trial or
context of jury decision at hand.
Our findings have practical implications for both
witness preparation and jury selection. In the present
study, we looked at factors associated with expert
witness confidence. Trial consultants can focus ef-
forts of witness preparation not only toward accurate
testimony, but also toward methods of bolstering
confidence. Boccaccini and colleagues
55
have under-
taken preliminary efforts in this area. The present
study offers an evidence-based list of verbal and
nonverbal factors that can be improved in witness
preparation. These include, but are not limited to, a
moderated, stable tone of voice; clarity in speech;
moderately paced speech; willingness to acknowl-
edge a degree of certainty; smooth narrative state-
ments; good posture; poise; consistent eye contact;
hearing accurately and responding appropriately;
and indication of credentials and knowledge.
The present study also has implications for trial
consultation. Trial consultants can advise legal coun-
sel to incorporate questions tapping the construct of
extroversion to assess jurors’ potential to resist the
impact of expert witness testimony. Example items
would include questions regarding leadership, gre-
gariousness, positive emotions, and social interactions.
A purpose of the present study was to arrive at a
definition of confidence suitable for psycholegal re-
search. Previous confidence scholars
11,19
have enu-
merated factors related to perceived confidence such
as speech style, arrogance, and nervousness. We es-
tablished working definitions of various levels of ma-
nipulated confidence cues based on flow and tone of
speech, certainty of conclusions, posture, eye con-
tact, and accurate responses to questions. Thus, the
behavioral definition included both verbal and non-
verbal components.
Limitations and Future Directions
Three areas of limitation to this study should be
noted. First, participants were mostly from the
Expert Witness Confidence
72 The Journal of the American Academy of Psychiatry and the Law
southeastern United States, of young average age,
predominantly female, Caucasian, and Christian.
Therefore, the conclusions drawn may be extended
to more diverse populations only with caution. Sec-
ond, the present study included only male expert
witnesses for the prosecution testifying about future
dangerousness. Although results may differ with fe-
male or expert witnesses for the defense, this parsi-
monious design was used to balance limits in gener-
alizability with methodological simplicity. Previous
empirical work also called into question whether ju-
rors pay attention to expert opinions on future dan-
gerousness.
56
Hence, the associations in the present
study should be replicated in a different testimony
context.
Also, a common concern in jury research is the use
of mock jurors. Although we must acknowledge this
limitation, psycholegal scholars have largely sup-
ported the use of mock jurors because of their simi-
larities to real jurors
57
and the use of such research in
many areas of legislative decision-making.
58
The
high rate of death-qualified mock jurors also war-
rants comment in relation to generalizability. Our
finding that approximately 299 of 317 (94.3%) were
death-qualified jurors differs from rates of both ac-
tual juror samples and other mock juror samples
(range from 64%–78%).
59–61
We suspect this find-
ing to be a geographic artifact, given the political
conservatism of the southeastern United States. Such
a limitation certainly must be taken into account
when drawing conclusions from this work.
Confidence research would benefit from employ-
ing a unified definition of confidence. Inquiry may
be also undertaken into the notion of pseudoconfi-
dence, or feigning confidence, to test how such a
variation of genuineness relates to credibility. The
present findings show that juror extroversion is an
essential trait in many aspects of jury interaction.
Further investigation of juror extroversion as it re-
lates to other aspects of decision-making (e.g., prej-
udice/bias, guilt/innocence, other types of crimes) is
warranted. Finally, future trial consultation research
should seek to develop evidence-based witness con-
fidence/efficacy scales, and evaluate cues of confi-
dence in the context of witness preparation.
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Expert Witness Confidence
74 The Journal of the American Academy of Psychiatry and the Law
