Physiological Differences: Stimulation Sensitivity
and Behavioral Systems
Eysenck argued that extraverts and introverts differ not only in terms of
behavior but also in their physiological makeup. He originally maintained
that extraverts and introverts have different levels of cerebral cortex arousal
when in a nonstimulating, resting state (Eysenck, 1967). Although it may
sound backward at first, he proposed that extraverts generally have a lower
level of cortical arousal than do introverts. Extraverts seek out highly arous-
ing social behavior because their cortical arousal is well below their desiredlevel when doing nothing. In a sense, highly extraverted people are simply try-
ing to avoid unpleasant boredom. Their problem is feeding their need for stim-
ulation. Introverts have the opposite problem. They typically operate at anabove-optimal cortical arousal level. These people select solitude and nonsti-
mulating environments in an effort to keep their already high arousal level
from becoming too aversive. For these reasons, extraverts enjoy a noisy partythat introverts can ’t wait to leave.
Unfortunately, a great deal of research has failed to uncover the different
levels of base-rate cortical arousal proposed by Eysenck. Introverts and extra-
verts do differ in how certain parts of their brains respond to emotional sti-
muli (Canli, 2004). However, they show no differences in brain-wave activity
when at rest or when asleep (Stelmack, 1990). But this does not mean that
Eysenck ’s original theorizing was entirely off base. Rather, there is ample
evidence that introverts are more sensitive to stimulation than extraverts
(Bullock & Gilliland, 1993; Stelmack, 1990; Swickert & Gilliland, 1998).
That is, introverts are more quickly and strongly aroused when exposed to
external stimulation. Introverts are more likely to become aroused when they
encounter loud music or the stimulation found in an active social encounter.
Introverts are even more responsive than extraverts when exposed to chemi-
cal stimulants, such as caffeine or nicotine.
As a result of these research findings, many researchers now describe
extraverts and introverts in terms of their different sensitivity to stimulation
rather than differences in cortical activity. However, the effect is essentially
the same. Because of physiological differences, introverts are more quicklyoverwhelmed by the stimulation of a crowded social gathering, whereas ex-
traverts are likely to find the same gathering rather pleasant. Extraverts are
quickly bored by slow-moving movie plots and soft music, but introvertsoften find these subtle sources of stimulation engaging.
Other researchers tie differences in extraversion and neuroticism to bio-
logically based differences in sensitivity to reinforcement. According to rein-
forcement sensitivity theory (Gray, 1982, 1987; Gray & McNaughton, 2000),
each human brain has a behavioral approach system (BAS) and a behavioral
inhibition system (BIS) . The exact regions of the brain and the specific pro-
cesses involved in each of these hypothetical systems remain to be deter-
mined. Nonetheless, like other persona lity concepts, individuals are said to
differ in the strength of these two systems and these individual differences
are relatively stable over time. People with a highly active BAS are intensely
motivated to seek out and achieve pleasurable goals. Compared to people226 CHAPTER 9 The Biological Approach