62 Genetic Basis of Personality Structure
phenotypic and psychometric analyses. However, numerous
psychometric studies have not resolved these problems, rais-
ing the possibility that studies of phenotypes alone may not
be sufficient.
The problem with phenotypic analyses is their reliance on
constructs that are by their nature fuzzy and imprecise. This
is illustrated by the confusion noted about the components of
extraversion (Depue & Collins, 1999; Watson & Clark,
1997). Conceptions of extraversion include sociability or af-
filiation (includes agreeableness, affiliation, social recogni-
tion, gregariousness, warmth, and social closeness), agency
(surgency, assertion, endurance, persistence, achievement,
social dominance, ascendancy, ambitiousness), activation
(liveliness, talkativeness, energy level, activity level, activity
level), impulsive–sensation seeking (impulsivity, sensation
seeking, excitement seeking, novelty seeking, boldness, risk
taking, unreliability, disorderliness, adventurousness, thrill
and adventure seeking, monotony avoidance, boredom sus-
ceptibility), positive emotions (positive affect, elatedness,
enthusiasm, exuberance, cheerfulness, merriness, joviality),
and optimism (Depue & Collins, 1999).
This list reveals the problems faced by attempts to delin-
eate phenotypic structure. Not only does the content of extra-
version differ across models, but the definition of each basic
or lower-order trait may also differ across models and mea-
sures. Moreover, the meaning of putatively distinct traits
overlaps so that facet traits defining a given domain shade
into each other and into facet traits defining other domains.
This fuzziness is probably an inevitable consequence of
using natural language concepts that evolved to capture so-
cially significant behaviors that are multidetermined. It adds
to concerns that the taxonomies of phenotypic traits may not
represent natural cleavages in the way behavior is organized
nor reflect underlying etiological structures.
This fuzziness contributes to the considerable variability
in personality phenotypes so that minor variations in mea-
sures and samples influence the number and contents of fac-
tors. The problem is compounded by the fact that many
decisions about methodology and analytic strategies have an
arbitrary component. More objective criteria are needed to
guide decisions on the number of higher-order domains and
the location of lower-order or basic traits within domains and
to define a systematic set of basic traits. Phenotypic analyses
are concerned primarily with describing trait covariation.
This evokes the oft-voiced criticism of the five-factor
approach—it is descriptive rather than explanatory. The
basic problem ofwhytraits are related to each other is not
considered. An understanding of etiology of trait covariance,
especially genetic etiology, would provide a conceptual foun-
dation for current models. At each level of the trait hierarchy,
traits and behaviors, including test items, could be grouped
according to a shared etiology. Etiology would provide an
additional criterion to supplement the usual psychometric cri-
teria such as proposed by Costa and McCrae (1997) to guide
decisions on the number and content of domains. Identifica-
tion of a robust model of personality structure would be facil-
itated by evidence that a given phenotypic structure reflects
the genetic architecture of personality traits. Unfortunately
there are few studies of the genetic architecture underlying
multiple personality traits compared to studies of phenotypic
structure. Evidence that a given phenotypic structure paral-
lels genotypic structure would support the validity and gener-
alizability of the structure.HERITABILITYThe foundation for an etiological understanding of personal-
ity structure and for a behavioral genetic approach is pro-
vided by evidence that genetic influences account for
approximately 40–60% of the variance for virtually all per-
sonality traits, with most of the remaining variance being ex-
plained by nonshared environmental effects (Bouchard,
1999; Loehlin & Nicholls, 1976; Plomin, Chipeur, &
Loehlin, 1990). The broad traits of extraversion and neuroti-
cism have received most attention. The data from several
twin studies yield heritability estimates of approximately
60% for extraversion and 50% for neuroticism. Loehlin
(1992) also examined multiple personality scales organized
according to the five-factor framework. Estimates of about
40% heritability were obtained for each domain. Subsequent
studies using the NEO-PI-R yielded heritability estimates of
41% for neuroticism, 53% for extraversion, 41% for agree-
ableness, and 40% for conscientiousness (Jang, Livesley,
Vernon, & Jackson, 1996; see also Bergeman et al., 1993;
Jang, McCrae, Angleitner, Riemann, & Livesley, 1998). Non-
additive genetic effects accounted for 61% the variance in
openness to experience.
Although the evidence points to a significant genetic com-
ponent to personality traits, it has been suggested that traits
could be divided into temperament traits that have a substan-
tial heritable component and character traits that are largely
environmental in origin. If this is the case and environmental
factors give rise to distinct traits, the role of genetic criteria in
clarifying trait structure would be limited. The evidence does
not, however, support the proposal. Putatively charactero-
logical traits such as openness to experience are as herita-
ble as so-called temperament traits. Moreover, molecular
genetic studies have found significant allelic associations be-
tween so-called character traits such as cooperativeness and