92 Biological Bases of Personality
stages of production of the monoamines and some of the en-
zymes (DBH, COMT, MAO) involved in the conversions
from one stage to another. The metabolite for dopamine is ho-
movanillic acid (HVA), for norepinephrine it is 3-methoxy-4-
hydroxyphenylglycol (MHPG), and for serotonin it is
5-hydroxyindoleacetic acid (5-HIAA).
Theorists are in fair agreement on the role of dopaminergic
systems in motivation based on studies of other species: ap-
proach and sensitivity to stimuli associated with reward
(Crow, 1977; Gray, 1982, 1987; Stein, 1978); foraging and ex-
ploration and positive emotions like hope, desire, and joy in
humans (Panksepp, 1982; Zuckerman, 1991); and novelty or
sensation seeking in animals and humans (Bardo, Donohew, &
Harrington, 1996; Cloninger et al., 1993; Le Moal, 1995;
Zuckerman, 1984, 1991). I have proposed that the activity of
the mesolimbic dopamine system is related to a broad ap-
proach trait that includes extraversion, sensation seeking, and
impulsivity (Zuckerman, 1991). Considering that dopaminer-
gic reactivity is also related to aggression and sexuality in
many species, it is also possible that the third dimension
of personality, low socialization, or psychoticism, may also
be involved. Gray’s (1987) model linked dopamine and re-
ward sensitivity with impulsivity, a dimension related to high
E, P, and N, although his more recent remarks (Gray, 1999)
suggest that he is linking dopamine more closely with the P
dimension because of this transmitter’s involvement in
schizophrenia.
Depue and Collins (1999) defined a broad view of extra-
version with two main factors: interpersonal engagement, or
affiliation and warmth, and agency, which includes social
dominance, exhibitionism, and achievement motivation. Pos-
itive affect and positive incentive motivation are more
strongly associated with the agentic extraversion factor. Im-
pulsivity and sensation seeking are regarded as constituting an
emergent factor representing a combination of extraversion
and constraint (a dimension related to Eysenck’s P and Costa
and McCrae’s conscientiousness). The “lines of causal neuro-
biological influence” are suggested to lie along the orthogonal
dimensions of extraversion and constraint rather than along
the dimension of impulsive sensation seeking. Although
Depue and Collins say that this structural system does not
mean that positive incentive motivation and its dopaminergic
basis are related only to extraversion, the expectation is that
they will be more strongly related to agentic extraversion than
to impulsive sensation seeking or constraint.
Only a few correlational studies of monoamine CSF
metabolites and personality traits were done prior to 1991
(Zuckerman, 1991), and they generally showed few signifi-
cant relationships between the dopamine metabolite HVA
and either extraversion or sensation seeking. This is still the
case with studies that simply correlate CSF levels of HVA
with questionnaire measures of extraversion, even when
there is sufficient power to detect weak relationships (Limson
et al., 1991). In fact, the Limson et al. study failed to find
any correlations between CSF metabolites of serotonin
(5-HIAA), norepinephrine (MHPG), norepinephrine itself,
and Dopac and any of the personality measures assessed by
the Minnesota Multiphasic Personality Inventory (MMPI),
Eysenck Personality Questionnaire (EPQ), or Cloninger’s
Temperament Character Inventory (TCI). As with psy-
chophysiological measures, levels of neurotransmitter activ-
ity in a resting basal state are not sensitive to variations in
personality, at least as the latter is measured in self-report
questionnaires. However, studies that attempt to potentiate or
attenuate activity in neurotransmitters with agonists or antag-
onists have yielded some significant findings in regard to per-
sonality, even though they typically use very small sample
sizes.
Depue, Luciana, Arbisi, Collins, and Leon (1994) chal-
lenged the dopamine system with bromocriptine, a potent
agonist at D 2 receptor sites, and measured the effects using
inhibition of prolactin secretion and activation of eye-blink
rate, two measures of dopamine activation. The correlations
between Positive Emotionality (PE) and baselline measures
of the dopamine activity indicators were small and insignifi-
cant, but they found significant correlations between the pu-
tative measures of dopamine response to the agonist and the
PE (an extraversion type measure) factor from Tellegen’s
MPQ. Rammsayer (1998, 1999) challenged Depue et al.’s
interpretation of their findings as indicative of higher
dopamine reactivity in high-PE persons (extraverts) than in
lows, suggesting that the prolactin response would indicate
just the reverse (i.e., higher reactivity in the low-PE persons).
The disagreements on the meaning of the data are too com-
plicated to elucidate here.
Rammsayer’s interpretation of the findings is supported by
PET measures of higher cerebral blood flow to the dopamine-
rich basal ganglia areas in introverts than in extraverts (Fischer,
Wik, & Fredrikson, 1997); but another PET study found no re-
lationship between E and dopamine binding in the basal gan-
glia (N. S. Gray, Pickering, & Gray, 1994), and still another
found a positive relationship with E (Haier et al., 1987). The
first two of these studies used normal controls as subjects
whereas the Haier et al. study used patients with Generalized
Anxiety Disorder, a possible confounding factor.
Rammsayer, Netter, and Vogel (1993), using an inhibiter
of tyrosine hydroxlase, thereby blockading dopamine synthe-
sis, found no difference between introverts and extraverts in