Universality of Trait Structure 73factor scores to be computed. Their method uses the follow-
ing equation:
y=−^1 x (3.2)wherey=factor score for the common genetic factor, ã=
the factor loadings of each variable on the genetic factor of
interest (i.e., the column vector of estimated path coefficients
that represent the correlations between the common genetic
or environmental factor and the observed measures), −^1 =
correlation matrix between all of the variables (i.e., the in-
verse of the correlation matrix of the observed measures),
andx=each person’s score or response to each of the vari-
ables (i.e., column vector of observed values on the mea-
sures). Other methods are also available to compute genetic
and environmental factor scores (Thomis et al., 2000).
Domain Content
As discussed earlier, the facet structure of several five-factor
domains is still unclear. The same behavioral genetic
approach used to define and measure basic trait scales could
also be applied to the delineation of domain content. The
unity of a domain is demonstrated by evidence that a single
common genetic factor influences all the facets composing
the domain. This approach could be used to clarify the loca-
tion of impulsivity within the higher-order structure. The
five-factor model locates impulsivity in Neuroticism,
whereas Eysenck places it within Extraversion. As noted
earlier, the bivariate correlations of this facet with other
Neuroticism facets assessed with the NEO-PI-R are lower
than correlations between other facets. Etiological data could
be used to relocate impulsivity with other traits with which it
shares a common etiology. Alternatively the item content
could be changed based on genetic and environmental etiol-
ogy so that correlations with the other Neuroticism facets are
increased (of the loadings on the common factors are in-
creased). In the case of the DAPP scales, impulsivity is part
of the phenotypic trait of stimulus seeking along with sensa-
tion seeking and recklessness. Multivariate genetic analyses
showed that a single common genetic factor underlies this
dimension that is defined by sensation seeking and reckless-
ness (see Table 3.5). Impulsivity has a low loading on the
factor and a substantial specific heritable component. It
appears that impulsivity as defined within the DAPP structure
is a specific heritable entity and not the result of interaction
between extraversion and constraint or psychoticism as sug-
gested by Depue and Collins (1999) or extraversion and psy-
choticism as suggested by Gray (1970, 1973, 1987; Pickering
& Gray, 1999), although it is consistent with Gray’s argument
that impulsivity is a fundamental dimension of temperament.
The findings of behavioral genetic studies of personality
structure also have implications for attempts to identify the
putative genes for personality. Most molecular genetic stud-
ies of personality use an analytic strategy that correlates a
totalpersonality trait score such as Neuroticism with varia-
tions in the candidate allele (Lesch et al., 1996). As the stud-
ies described show, the total scale score confounds multiple
genetic and environmental effects and reduces the power to
detect putative loci. The use of etiological factor scores that
index the proportions of the personality phenotype directly
attributable to specific genetic and environmental effects
(Boomsma, 1996; Sham et al., 2001; Thomis et al., 2000)
could reduce these confounds.UNIVERSALITY OF TRAIT STRUCTUREMost models of personality traits including Eysenck’s three-
factor model (Eysenck & Eysenck 1992), the five-factor
model, and diagnostic categories of personality disorder
proposed in the DSM-IV(American Psychiatric Association,
1994) assume that the taxonomies proposed reflect a univer-
sal structure. This assumption is also assumed to apply to the
measures developed to assess these constructs. The only dif-
ferences that these models of personality (and their mea-
sures) permit between cultures and other groups (e.g.,
gender) are quantitative in nature; they typically mean differ-
ences in trait levels or severity. If these assumptions are cor-
rect, we should find that the etiological architecture of
personality is also invariant across cultures and other basic
groupings. We discuss this idea with respect to cross-cultural
comparisons and the effects of gender.Cross-Cultural ComparisonsMultiple studies show that the observed factorial structure of
scales such as the NEO-PI-R is stable across cultures. For ex-
ample, McCrae and Costa (1997) reported that the five-factor
structure is consistent across samples from the United States,
Western Europe, and Asia (see also Costa & McCrae, 1992;
McCrae et al., 2000). The issue of cross-cultural stability also
applies to etiological structure. Earlier, we described fitting
an independent pathways model to the six facets defining
NEO-PI-R domains in independent samples of German and
Canadian twins. The universality of genetic effects can be
evaluated by testing the equivalence of the genetic and envi-
ronmental structures across independent samples. It is possi-
ble to test whether: (a) the same genetic and environmental
factors influenced the Canadian and German samples; and
(b) whether these factors influenced each sample to the same