426 Part V Biological/Evolutionary Theories
correspond well with their greater spontaneity, social disinhibition, and impulsive-
ness. In a study by Cynthia Doucet and Stelmack (2000), however, it was only
motoric response rate—not cognitive processing speed—that differentiated intro-
verts and extraverts. Extraverts were faster motorically but not cognitively. Extra-
verts may move faster but they do not think faster than introverts.
Optimal level of arousal is another of Eysenck’s hypotheses that has generated
some research. Eysenck theorized that introverts should work best in environments of
relatively low sensory stimulation, whereas extraverts should perform best under con-
ditions of relatively high sensory stimulation (Dornic & Ekehammer, 1990). In an
important study conducted by Russell Geen (1984), introverted and extraverted par-
ticipants were randomly assigned to either a low noise or high noise condition and
then given a relatively simple cognitive task to perform. Results showed that introverts
outperformed extraverts under conditions of low noise, whereas extraverts outper-
formed introverts under conditions of high noise. These findings not only support
Eysenck’s theory but also suggest that people who prefer to study in public places
(like a dorm study area) are more likely to be extraverts. Introverts, on the other hand,
find such noisy environments distracting and therefore tend to avoid them.
A second source of support for Eysenck’s biologically based theory of per-
sonality comes from behavioral genetics. Behavioral genetic research is often based
on the study of twins, both identical and fraternal, who have been raised together
or apart. Twin studies have found that most basic personality traits have heritabil-
ity estimates of between 40% and 60% (Plomin & Caspi, 1999). In other words,
an individual’s genetic makeup goes about halfway toward explaining his or her
basic traits. For instance, the trait of extraversion, or outgoingness, often correlates
around .50 for identical twins and around .20 to .25 for fraternal twins, which leads
to a heritability estimate of between 50% and 60%. Likewise, between 50% and
55% of the difference in neuroticism is due to genetics (Bouchard & Loehlin, 2001;
Caspi, Roberts, & Shiner, 2005; Krueger & Johnson, 2008; Plomin & Caspi, 1999).
Moreover, other researchers have begun to uncover specific gene locations involved
in producing neurotransmitters, such as serotonin, that are connected to traits of
extraversion, neuroticism, and psychoticism (Gillespie et al., 2008).
In summary, research tends to support Eysenck’s notion that personality fac-
tors have a biological basis and are not simply dependent on what we have learned.
Indeed, consistent with a biological basis of personality, the major traits appear to
be consistent in most countries of the world (McCrae, 2002; Poortinga, Van de
Vijver, & van Hemert, 2002). How and when personality traits are expressed is
clearly influenced by our cultural and social context. But that we all can be described
on similar dimensions of personality (e.g., extraversion or neuroticism) is influenced
by our biological makeup. Personality, in short, is molded by both nature and nurture.The Biological Basis of Neuroticism
Recall that Eysenck (1967) postulated that neuroticism resulted from increased
activity or responsiveness and lower activation thresholds in the limbic system. This
pattern is the same as introversion but the location is different: introversion involves
increased activity and lower thresholds for cortical and reticular arousal rather than
limbic system arousal. The limbic system is comprised of the subcortical structures