94 Biological Bases of Personality
moderately heritable (41%) in females (Harris, Vernon, &
Boomsa, 1998). In rats T has reward effects in the nucleus ac-
cumbens, the major site of dopaminergic reward. Administra-
tion of a dopamine receptor blocker eliminates the rewarding
effects of T in rats, suggesting that its rewarding effects are
mediated by an interaction with dopamine in the mesolimbic
system (Packard, Schroeder, & Gerianne, 1998).
The hormone T affects personality traits and may account
in part for many of the personality trait differences between
men and women. Men and women do not differ on the pure so-
ciability or affiliative type of extraversion, but they do on the
agentic type, which includes dominance, assertiveness, sur-
gency, and self-confidence. To the extent that sensation seek-
ing is associated with extraversion, it is with the agentic type.
Daitzman and Zuckerman (1980) found that T in young
males was positively correlated with sociability and extraver-
sion, as well as with dominance and activity and inversely with
responsibility and socialization, indicating an association with
the agentic type of extraversion. Windle (1994) also found that
testosterone was associated with a scale measuring behavioral
activation, characterized by boldness, sociability, pleasure
seeking, and rebelliousness. Dabbs (2000) also found that T is
associated with a type of extraversion characterized by high
energy and activity levels and lower responsibility.
Summary
Eysenck’s theory relating cortical arousal to extraversion has
been extensively tested using the EEG and, in more recent
times, the brain scanning methods. The EEG studies yielded
mixed results in which the sources of differences between stud-
ies were not clearly apparent. Two cerebral blood flow studies
did confirm that extraverts were cortically underaroused re-
lated to introverts in female subjects but not in males. Studies
measuring cortical arousability have also not clarified the pic-
ture. Apparently, experimental conditions affecting attention
or inhibition may confound the relationship with E. Some more
consistent results have been obtained from EP studies of re-
sponses at subcortical levels in which conscious attention is
less of a factor. Although Eysenck’s theory is confined to corti-
cal arousal and reactivity, differences between introverts and
extraverts have been found at lower levels of the central ner-
vous system, even in a spinal motoneuronal reflex.
Theories of the biochemical basis of extraversion have fo-
cused on the monoamine neurotransmitters, particularly
dopamine. Simple correlational studies between the
monoamine metabolites and trait measures of E have not
yielded significant findings, although there is some evidence
that drugs that increase noradrenergic or serotonergic activity
in depressed patients also increase their extraversion and
sociability. This may be an indirect effect of the reduction in
depression rather than a direct effect on E. The enzyme
MAO-B is involved in regulation of the monoamines, partic-
ularly dopamine. Low levels of MAO have been related to
arousal and activity in newborn human infants and to socia-
ble behavior in adult humans and monkeys. These results
suggest that a dysregulation of the dopamine system may be
a factor in extraversion even in its earliest expression in the
behavior of newborns. The hormone testosterone is related to
E, but more so to E of the agentic type, which is the type
characterized by dominance, assertiveness, surgent affect,
high energy levels, activity, and irresponsibility, rather than
simple sociability and interest in social relationships. This
distinction between the two types of E has been hypothesized
to be crucial for the relationship between dopamine and E as
well (Depue & Collins, 1999).NEUROTICISM/ANXIETY/HARM AVOIDANCEAlthough the broad trait of neuroticism/anxiety includes
other negative emotions, such as depression, guilt, and hostil-
ity, and character traits such as low self-esteem, neuroticism
and anxiety are virtually indistinguishable as traits. Neuroti-
cism is highly correlated with measures of negative affect,
but when the negative affect was broken down into anxiety,
depression, and hostility components, anxiety had the highest
correlation, and hostility the lowest, with the N factor while
depression was intermediate (Zuckerman, Joireman, Kraft, &
Kuhlman, 1999). Hostility had a higher relationship to a fac-
tor defined by aggression.
Eysenck (1967) assumed a continuity between N as a per-
sonality trait and anxiety disorders. Indeed, N is elevated in
all of the anxiety and depressive mood disorders, and longi-
tudinal studies show that the trait was evident in most persons
before they developed the symptoms of the clinical disorder
(Zuckerman, 1999). In the first half of the twentieth century,
when little was known about the role of the limbic system in
emotions, the biological basis of neuroticism and anxiety trait
was related to overarousal or arousability of the sympathetic
branch of the autonomic nervous system. Such arousal is ap-
parent in state anxiety elicited by anticipation of some kind of
aversive stimulus or conditioned stimuli associated with
aversive consequences.
Autonomic overarousal is apparent in the primary symp-
toms of many anxiety disorders. On the assumption of conti-
nuity between the N trait and these disorders, it was expected
that autonomic arousal, as assessed by peripheral measures
such as heart rate (HR), breathing rate (BR), blood pressure
(BP), and electrodermal activity (EDA), would be correlated
with N. In Eysenck’s (1967) theory, N was ultimately based