Extraversion/Sociability 91Snidman, 1988). Shyness and inhibition, however, are traits
that are a mixture of introversion and neuroticism or anxiety;
therefore, the correlation with HR could be due to the anxiety
component rather than to E.
Eysenck’s model for the trait of extraversion produced a
great deal of research in the area of psychophysiology. But
psychophysiology has its problems as a branch of neuro-
science. Both tonic and phasic psychophysiological measures
are highly reactive to environmental conditions. Tonic levels
can vary as a function of reactions to the testing situation it-
self, and phasic reactions depend on the specific qualities of
stimulation such as intensity and novelty. It is not surprising
that the relationships of physiological measures with person-
ality traits often interact with these stimulus characteristics in
complex ways. Eysenck’s theory based on optimal levels of
stimulation has received some support. Those based on dif-
ferences in basal arousal levels are beginning to receive some
support from PET studies, although the earlier results with
EEG measures remain problematic.
Monoamines
The monoamine neurotransmitter systems in the brain have
been the focus of most biosocial theories of personality. The
reasons are the evidence of their involvement in human emo-
tional and cognitive disorders and basic emotional and moti-
vational systems in other species. Much of the work with
humans has been correlational, comparing basal levels of the
neurotransmitters, as estimated from levels of their metabo-
lites in cerebrospinal fluid (CSF), blood, or urine, to person-
ality traits as measured by questionnaires. Of these sources
CSF is probably the best because the CSF is in direct contact
with the brain. But the indirect relationship of these indica-
tors with brain levels of activity (which can differ in different
brain loci) and the fact that some of the metabolites in plasma
and urine are produced in the peripheral nervous system
make the putative measures of brain amine activity problem-
atic. New imaging methods may eventually overcome these
problems by directly viewing the monoamine activities in
the brain itself. Added to these problems of validity of mea-
surement is the use of small numbers of subjects in most
studies, as well as the use of subjects with certain types of
disorders rather than normal subjects. The ethical constraints
of giving drugs that affect activity in the brain systems is an-
other barrier, although some of the more recent studies have
used such drugs in normals.
The freedom of investigators to experiment directly with
the brain in other species has given us a fairly coherent picture
of the emotional and motivational functions of the
monoamine systems in the brain, and bottom-up theorists
have used these findings to extend animal models to human
motivations and personality (Gray, 1982, 1987; Mason, 1984;
Panksepp, 1982; Soubrié, 1986; Stein, 1978). Top-down the-
orists have drawn on these findings from the comparative re-
search but have attempted to reconcile them with the relevant
research on humans, including clinical and personality studies
(Cloninger, Svrakic, & Prszybeck, 1993; Depue & Collins,
1999; Netter, Hennig, & Roed, 1996; Rammsayer, 1998;
Zuckerman, 1991, 1995). The problem with building a bridge
from two banks is to make it meet in the middle. With these
caveats let us first examine the case for extraversion.
The primary monoamines in the brain are norepinephrine,
dopamine, and serotonin. The first two are labeled cate-
cholamines because of the similarities in their structures.
Serotonin is an indoleamine. These are not independent neu-
rotransmitter systems because activity in one may affect ac-
tivity in another. Serotonin, for example, may have
antagonistic effects on the catecholamines. These kinds of
interaction must be kept in mind because most studies relate
one neurotransmitter to one personality trait. Some models
suggest that this kind of isomorphism of trait and transmitter
is the rule. This is a new kind of phrenology based on bio-
chemistry rather than bumps on the head.
To understand the human research one needs to know the
pathways of biosynthesis and catabolism (breakdown) of the
monoamines because some experiments block the precursors
of the transmitter to see its effect on behavior and most use
metabolite products of the catabolism to gauge activity in
the systems. Figure 4.2 is a simplified diagram showing theFigure 4.2 Biosynthesis and breakdown of the monoamines dopamine,
norepinephrine, and serotonin.
Note. COMT catechol-O-methyltransferase; MAO monoamine oxi-
dase; HVA homovanillic acid; DBH dopamine ß-hydroxylase; MHPG
3-methoxy-4-hydroxyphenylglycol; 5-HIAA 5-hydroxyindoleacetic acid.
FromPsychobiology of personality,p. 177, by M. Zuckerman, 1991,
Cambridge: Cambridge University Press. Copyright 1991 by Cambridge
University Press. Reprinted by permission.