86 Biological Bases of Personality
Figure 4.1 Two pathways to individual differences in personality: the
biological and the social.
mechanisms of learning combine to produce behavioral
traits. These traits are usually specific to certain types of situ-
ations. Depending on their generality and strength they com-
bine to form what we call personality traits.
Both of these pathways have a historical origin in the evo-
lutionary history of the species. Genetic changes account
for the origin and changes (over long periods of time) in
the species. Cultures represent the collective solutions of the
human species to the basic demands of evolution: survival
and reproduction. Cultural evolution is more rapid than bio-
logical evolution. Significant changes can occur within a gen-
eration, as with the sudden impact of computer technology on
the current generation.
This chapter describes the biological pathway up to, but
not including, conditioning. For each of four dimensions of
personality I describe theory and research at each level of
analysis along this pathway starting at the top (physiology).
At the genetic level I describe primarily the studies of mole-
cular genetics that link specific genes to traits. The biometric
genetic studies are covered in the chapter by Livesly, Jang,
and Vernon in this volume. The molecular studies link genes
more directly to the neurological and biochemical levels on
the way up to personality traits. An analysis of this type was
conducted a decade ago (Zuckerman, 1991). Advances occur
rapidly in the neurosciences. Ten years is equivalent to at
least several decades in the social sciences. I have made an
attempt to survey the changes since my last attempt. In a
chapter I can hope only to highlight some of these advances
and will reserve a more thorough review for a revision of my
1991 book. My approach draws heavily on comparative stud-
ies of other species as any psychobiological model must do
(Gosling, 2001; Zuckerman, 1984, 1991), but I cannot do so
within the constraints of a single chapter. I will limit compar-
ative studies to those in which there are clear biological
markers in common between animal and human models.TEMPERAMENT AND PERSONALITY TRAITSResearchers of temperament in children and behavioral traits
in other species have typically included certain dimensions
like emotionality, fearfulness, aggressiveness, approach ver-
sus withdrawal (in reactions to novel stimuli), general activ-
ity, playfulness, curiosity, sociability versus solitariness, and
inhibition versus impulsivity (Strelau, 1998). From the 1950s
through the 1970s personality trait classification was domi-
nated by two models: Eysenck’s (1947) three-factor theory
(extraversion, neuroticism, and psychoticism) and Cattell’s
(1950) 16-factor model. Eysenck’s (1967) model was biolog-
ically based with an emphasis on genetics, physiology, and
conditioning. Gray’s (1982, 1987) model is a bottom-up
model that starts with behavioral traits in animals and extrap-
olates to human personality. He places his three behavioral
dimensions (anxiety, impulsivity, fight-flight) within the axes
of Eysenck’s dimensions, but not lying on the axes of those
dimensions or being precise equivalents of them.
The first five-factor model originated in lexical studies of
trait-descriptive adjectives in language done in the 1960s
(Norman, 1963; Tupes & Christal, 1961) with its roots in a
much earlier study by Fiske (1949). Interest in this model
reawakened in the 1980s (Digman & Inouye, 1986; Goldberg,
1990; Hogan, 1982; McCrae & Costa, 1985). Most of these
studies used adjective rating scales. The translation of the
model into a questionnaire form (NEO-PI-R; Costa &
McCrae, 1992a) increased the use of the scales by personality
investigators. The five factors incorporated in this tests are la-
beled extraversion, neuroticism, agreeableness, conscientious-
ness, and openness to experience. The five factors have been
replicated in studies in many countries although with some
differences—particularly on the last factor, openness. The en-
thusiasts for the Big Five insist it is the definitive and final
word on the structure of personality (Costa & McCrae, 1992b),
although critics regard this claim as premature (Block, 1995;
Eysenck, 1992; Zuckerman, 1992). One of the criticisms of the
model is its atheoretical basis in contrast to Eysenck’s devel-
opment of his factors from theory as well as empirical factor