Chapter 14 Eysenck’s Biologically Based Factor Theory 417bimodally, distributed. Extraversion, for example, is fairly normally distributed in
much the same fashion as intelligence or height. That is, most people are near the
center of a bell-shaped distribution of extraversion. Eysenck contended that each
of these factors meets his four criteria for identifying personality dimensions.
First, strong psychometric evidence exists for each, especially Factors E and
N. The P factor (psychoticism) emerged later in Eysenck’s work but was not taken
seriously by other researchers until the mid-1990s (Eysenck, 1997b). Extraversion
and neuroticism (or anxiety) are basic factors in nearly all factor analytic studies
of human personality, including various versions of the Five-Factor Theory
(McCrae & Costa, 1999, 2002; John & Srivastava, 1999).
Second, Eysenck (1994a, 1994b) argued that a strong biological base exists
for each of his three superfactors. At the same time, he claimed that traits such as
agreeableness and conscientiousness, which are part of the five-factor taxonomy
(John, 1990; W. T. Norman, 1963; Tupes & Christal, 1961), do not have an under-
lying biological foundation.
Third, Eysenck’s three personality dimensions make sense theoretically. Carl
Jung (see Chapter 4) and others have recognized the powerful effect on behavior
of extraversion and introversion (Factor E), and Sigmund Freud (see Chapter 2) empha-
sized the importance of anxiety (Factor N) on shaping behavior. In addition, psy-
choticism (Factor P) agrees with theorists, such as Abraham Maslow (see Chapter 9),
who propose that psychological health ranges from self-actualization (a low P score)
to schizophrenia and psychosis (a high P score).
Fourth, Eysenck repeatedly demonstrated that his three factors relate to such
social issues as drug use (Eysenck, 1983), sexual behaviors (Eysenck, 1976), crim-
inality (Eysenck, 1964, 1998b; Eysenck & Gudjonsson, 1989), preventing cancer
and heart disease (Eysenck, 1991c, 1991d; Grossarth-Maticek, Eysenck, & Vetter,
1988), and creativity (Eysenck, 1993).
Extraversion
In Chapter 4, we explained that Jung conceptualized two broad personality types,
called “extraversion” and “introversion.” We also noted some differences between
his definitions and the prevailing notion of these two terms. Jung saw extraverted
people as having an objective or nonpersonalized view of the world, whereas intro-
verts have essentially a subjective or individualized way of looking at things.
Eysenck’s concepts of extraversion and introversion are closer to the popular usage.
Extraverts are characterized primarily by sociability and impulsiveness but also by
jocularity, liveliness, quick-wittedness, optimism, and other traits indicative of peo-
ple who are rewarded for their association with others (Eysenck & Eysenck, 1969).
Introverts are characterized by traits opposite those of extraverts. They can
be described as quiet, passive, unsociable, careful, reserved, thoughtful, pessimis-
tic, peaceful, sober, and controlled. According to Eysenck (1982), however, the
principal differences between extraversion and introversion are not behavioral, but
rather biological and genetic in nature.
Eysenck (1997a) believed that the primary cause of differences between extra-
verts and introverts is one of cortical arousal level, a physiological condition that is
largely inherited rather than learned. Because extraverts have a lower level of cortical