72 Genetic Basis of Personality Structure
content of domains. It also explains Costa and McCrae’s in-
sistence that domains are equal in breadth. If they are not, the
five-factor model is open to the criticism that the model is not
sufficiently parsimonious, as argued by Eysenck. This theo-
retical structure is understandable if trait taxonomies are con-
ceptualized only as lexical structures. It is possible, however,
that traits at the biological level are not organized in the sys-
tematic way proposed by the five-factor model.
There are no a priori reasons to assume that all basic traits
must be organized into a hierarchy or that each higher-order
domain is equally broad and defined by an equal number of
facets as hypothesized by the five-factor model. An equally
plausible model is that traits are organized into clusters that
differ in the number of basic traits that they subsume and that
the hierarchy is incomplete, with some specific traits showing
minimal degrees of covariation. This structure is illustrated
by the findings regarding the structure of the higher-order di-
mension of compulsivity identified in studies of personality
disorder traits (Livesley et al., 1998). Pathways models iden-
tified a single genetic dimension underlying the specific traits
that define this construct. Factor analyses show that it is con-
sistently not related to other traits—hence, the three pheno-
typic traits that delineate compulsivity from separate
higher-order factors. Compulsivity is, however, a trait nar-
rower than other higher-order domains. It appears to repre-
sent a distinct basic or lower-order trait based on a single
genetic dimension that does not have a hierarchical relation-
ship with other basic traits.
Basic-Level Traits: Defining the Basic
Unit of Personality
The idea that personality is inherited as a few genetic mod-
ules with broad effects and a large number of modules with
more specific effects focuses attention on the significance of
lower-order or basic traits. These findings are similar to eval-
uations of hierarchical models of cognitive ability that also
provide evidence that specific abilities are heritable (Casto,
DeFries, & Fulkner, 1995; Pedersen, Plomin, Nesselroade, &
McClearn, 1992). Basic traits do not appear to be specific
exemplars of the higher-order traits that they define or blends
of two or more factors (Hofstee, DeRaad, & Goldberg, 1992).
Rather, they are discrete genetic entities with their own bio-
logical basis. This suggests that personality models that re-
duce traits to a few global domains do not reflect the genetic
architecture of normal or disordered personality. As noted
earlier, personality research has tended to neglect these traits
in favor of more global dimensions. Yet evidence of speci-
ficity of genetic effects suggests that the basic traits are the
fundamental building blocks of personality that are more
important for understanding personality than are the global
constructs that have traditionally been the focus of research
and explanation. This approach again raises the question of
how basic traits should be conceptualized and defined, as
well as which criteria are relevant to defining domains.
Costa and McCrae (1998) noted the challenges of delin-
eating a comprehensive set of basic traits. The specificity of
genetic effects also reveals the challenge involved because of
the large number of genetic dimensions that are likely to be
involved. A genetic perspective does, however, provide a de-
finition of a basic dimension that could facilitate the identifi-
cation and assessment of these traits. The usual psychometric
criteria used to develop homogeneous scales could be sup-
plemented with the genetic criterion that a basic trait scale
represents a single specific genetic dimension. With this ap-
proach, items assessing a basic trait would form a genetically
homogeneous unit as opposed to a factorially homogeneous
unit. Items could then be selected according to their correla-
tion with the underlying genetic dimension. Thus items form-
ing a scale would share the same general and specific genetic
etiology. With this approach, the goal would be to use behav-
ioral genetic techniques to bring about definitions of the phe-
notype that correspond to what Farone, Tsuang, and Tsuang
(1999) refer to as “genetically crisp categories” (p. 114).
An example of this approach is provided by a study of
the genetic structure of the Eysenck Personality Question-
naire (Heath, Eaves, & Martin, 1989). This instrument has
three broad scales composed of 21 to 25 items that assess
Neuroticism, Extraversion, and Psychoticism. Heath and col-
leagues extracted a common genetic and environmental
factor for Neuroticism and Extraversion, indicating that these
items are etiologically homogeneous. In contrast, little evi-
dence was found for a common genetic factor for the Psy-
choticism items. Subsequent analyses showed that the items
formed into two distinct genetic factors: paranoid attitudes
and hostile behavior. The results of such a systematic evalua-
tion of item etiology could be used to form etiologically
homogeneous scales.
This approach could be used either to develop new scales
or modify existing scales so that they resemble the underly-
ing genetic architecture more closely. This could be achieved
by applying differential weights that index the influence of
specific genetic and environmental influences on different
traits. In this way, questions about the phenotypic structure of
personality are addressed, and scales could be constructed so
that they do not reflect competing genetic and environmental
influences.
The estimation of genetic and environmental factor scores
is a relatively new and active area of research. Sham et al.
(2001) recently described a method that permits these genetic