418 Part V Biological/Evolutionary Theories
arousal than do introverts, they have higher sensory thresholds and thus lesser reactions
to sensory stimulation. Introverts, conversely, are characterized by a higher level of
arousal, and as a result of a lower sensory threshold, they experience greater reactions
to sensory stimulation. To maintain an optimal level of stimulation, introverts, with
their congenitally low sensory threshold, avoid situations that will cause too much
excitement. Hence, introverts shun such activities as wild social events, downhill skiing,
skydiving, competitive sports, leading a fraternity or sorority, or playing practical jokes.
Conversely, because extraverts have a habitually low level of cortical arousal,
they need a high level of sensory stimulation to maintain an optimal level of stimu-
lation. Therefore, extraverts participate more often in exciting and stimulating activ-
ities. They may enjoy such activities as mountain climbing, gambling, driving fast
cars, drinking alcohol, and smoking marijuana. In addition, Eysenck (1976) hypoth-
esized that extraverts, as opposed to introverts, will engage in sexual intercourse
earlier, more frequently, with a wider range of partners, in a greater number of
positions, with a larger variety of sexual behaviors, and will indulge in longer pre-
coital love play. Because extraverts have a lower level of cortical arousal, however,
they become more quickly accustomed to strong stimuli (sexual or otherwise) and
respond less and less to the same stimuli, whereas introverts are less likely to become
bored and uninterested in routine activities carried on with the same people.Neuroticism
The second superfactor extracted by Eysenck is neuroticism/stability (N). Like
extraversion/introversion, Factor N has a strong hereditary component. Eysenck
(1967) reported several studies that have found evidence of a genetic basis for such
neurotic traits as anxiety, hysteria, and obsessive-compulsive disorders. In addition,
he found a much greater agreement among identical twins than among fraternal
twins on a number of antisocial and asocial behaviors such as adult crime, child-
hood behavior disorders, homosexuality, and alcoholism (Eysenck, 1964).
People who score high on neuroticism often have a tendency to overreact emo-
tionally and to have difficulty returning to a normal state after emotional arousal. They
frequently complain of physical symptoms such as headache and backache and of
vague psychological problems such as worries and anxieties. Eysenck (1967, 1982)
proposed this emotional reactivity in neuroticism is due to having a highly reactive
limbic system, including the amgydala and hypothalamus. Neuroticism, however, does
not necessarily suggest a neurosis in the traditional meaning of that term. People can
score high on neuroticism and be free of any debilitating psychological symptoms.
Eysenck accepted the diathesis-stress model of psychiatric illness, which sug-
gests that some people are vulnerable to illness because they have either a genetic or
an acquired weakness that predisposes them to an illness. This predisposition (diathesis)
may interact with stress to produce a neurotic disorder. Eysenck assumed that people
at the healthy end of the N scale have the capacity to resist a neurotic disorder even in
periods of extreme stress. High N scorers, however, may suffer a neurotic reaction as
a result of only a minimal level of stress. In other words, the higher the neuroticism
score, the lower the level of stress necessary to precipitate a neurotic disorder.
Because neuroticism can be combined with different points on the extraversion
scale, no single syndrome can define neurotic behavior. Eysenck’s factor analytic