Extraversion/Sociability 93either baseline dopamine or reactivity to the blockading
agent. Despite the lack of difference in dopaminergic activity
or reactivity, they found that reaction time performance was
markedly impaired in introverts but not in extraverts by the
dopamine blockading agent. In another study, using a chem-
ical that selectively blocks D 2 receptors and inhibits
dopamine neurons in the limbic and cortical regions of the
brain, Rammsayer (1998) again found a detrimental effect on
reaction (liftoff) time in introverts but not in extraverts. The
agent that was used caused a marked decrease in alertness
and cortical arousal, but this effect was equivalent in intro-
verts and extraverts. Both this finding and the performance
findings would seem to contradict Eysenck’s arousal expla-
nation for the differences between introverts and extraverts.
That theory would predict a more detrimental effect in ex-
traverts because they supposedly start with a lower level of
cortical arousal. But the results also raise the question, What
is the source of the performance differences between intro-
verts and extraverts if they do not differ in dopamine activity
or reactivity?
The answer might lie in the interactions of dopaminergic
and other neurotransmitters or hormones or, at another level,
in the genetics of the dopaminergic receptors. Considerable
interest has developed in a gene associated with the dopamine
receptor 4 (DRD4). Allelic variations in this gene have been
associated with novelty or sensation seeking, but not with ex-
traversion (Ebstein, Nemarov, Klotz, Gritsenko, & Belmaker,
1997; Ebstein et al., 1996).
Simple correlative studies have found no relationship be-
tween serotonin or norepinephrine and E or other personality
variables measured by questionnaires given to adult subjects.
A study using CSF from newborns in predicting tempera-
mental traits found that infants born with low levels of
the serotonin metabolite 5-HIAA showed low sociability at
9 months of age (Constantino & Murphy, 1996). Retest relia-
bility for 5-HIAA in neurologically normal infants was very
high(r=.94).
A study of adults with depressive disorder treated with
either a noradrenergic or a serotonergic reuptake inhibiter,
which increase activity in those systems, showed that there
were significant increases in measures of E and gregarious-
ness (sociability) in those treated with these drugs (Bagby,
Levitan, Kennedy, Levitt, & Joffe, 1999). The change in E was
correlated with the change in depression severity, but the
change in sociability was not. Although the result with socia-
bility probably represents a change of state rather than the
preillness trait, serotonin and norepinephrine might play some
role in the trait as well. Studies of serotonin transporter genes
have not shown any relationship to E, although they have to
other personality traits (Hamer, Greenberg, Sabol, & Murphy,
1999; Jorm, Henderson, Jacomb, Croft, & Easteal, 1997).Monoamine OxidaseMonoamine oxidase (MAO) is an enzyme involved in the
catabolic deamination of monoamines. Evidence using selec-
tive monoamine inhibitors suggests that MAO-Type B, as-
sayed from blood platelets in humans, is preferentially
involved in the catabolic breakdown of dopamine more than
the other two brain monoamines, norepinephrine and
dopamine (Murphy, Aulakh, Garrick, & Sunderland, 1987).
Although no direct correlation of platelet and brain MAO has
been found, indirect assessments and the effects of MAO in-
hibitors on depression, as well as a large body of behavioral
data, suggest that there must be a connection, if only one lim-
ited to certain brain areas. Platelet MAO is normally distrib-
uted in the human population, is highly reliable although it
increases in brain and platelets with age, and is lower in men
than in woman at all ages, and variations are nearly all ge-
netic in origin. Unlike other biochemical variables it does not
vary much with changes in state arousal. Thus, MAO has all
of the characteristics of a biological trait.
Low levels of MAO-B taken from umbilical cord blood
samples in newborn infants were related to arousal, activity,
and good motor development (Sostek, Sostek, Murphy,
Martin, & Born, 1981). High levels of the enzyme were re-
lated to sleep time and general passivity. The relationship
with motor development is particularly suggestive of devel-
opment of the dopamine-influenced basal ganglionic areas of
the brain involved in motor coordination. In a study of mon-
keys living in a colony in a natural environment, low-platelet
MAO was related to high sociability, activity, dominance, and
sexual and aggressive activity, a broad array of E-type traits
described by Depue and Collins (1999) as agentic extraver-
sion. However, in human correlative studies the results relat-
ing MAO-B to questionnaire-measured extraversion have
been inconsistent (Zuckerman, 1991). The enzyme has more
consistently correlated (inversely) with the trait of sensation
seeking. But using reported behavioral indices of sociability
in college students, low MAO was related to sociability and
high MAO to social insolation (Coursey, Buchsbaum, &
Murphy, 1979).HormonesThe hormone testosterone (T) is produced by both men and
women but is 8 to 10 times as high in men as in women.
Plasma T is highly heritable (66%) in young adult males and