74 Genetic Basis of Personality Structure
degree. Two tests of equivalency were applied. The first eval-
uated equivalency of model formby testing the hypothesis
that the same kind and number of genetic parameters are
required to explain the data across the two samples. Sample
differences are hypothesized to be limited to differences in
the magnitude of the genetic and environmental influence
exerted on a domain’s facet scales. If equivalence of model
form was supported across the samples, the next step was to
evaluate the magnitudeof genetic and environmental influ-
ences across samples. This was accomplished by applying a
model with the same parameters to both samples. That is, the
model specified the same number and type of factors in both
samples and identical and constrained the factor loadings to
be identical.
The results of tests of model form and magnitude for
NEO-PI-R Neuroticism are shown in Table 3.6. The same
number and types of genetic and environmental influences
(two additive genetic and two nonshared environmental com-
mon factors) were identified in both samples, suggesting that
the structure of neuroticism was similar across the samples.
When the factor loadings on the common factors from the
German sample were made to be the same as those on the
Canadian sample (and vice versa), the model no longer fit
the data. The results suggested that the primary differences
between the German and Canadian samples were limited to
the magnitude rather than kind of genetic and environmental
effects supporting the claim that the factorial structure of the
NEO-PI-R facets is universal.
Gender Differences
Personality tests are usually constructed to minimize gender-
based differences by eliminating items whose intercorrela-
tions with the other items can be attributable to gender and
eliminating items evoking marked gender differences in
endorsement. The approach yields scales that are applicable
to both females and males but it overlooks the possibility of
gender differences in the etiology. Behavioral genetic meth-
ods may be used to determine whether the same genetic and
environmental factors influence personality measure scores
in males and females and whether the etiological architecture
underlying the factorial structure of a personality measure is
the same in males and females.
The first question can be answered by fitting sex-limitation
models to personality data (Neale & Cardon, 1992). This is
accomplished by fitting a simple extension of the usual heri-
tability model that uses data from same- and opposite-sex
twin pairs to test whether the same genetic factors operate in
males and females. In this case, gender differences are limited
to differences in the magnitude of genetic and environmental
influences. Another form of sex-limited gene expression oc-
curs whendifferentgenes control the expression of a trait that
is measured in the same way in males and females. With this
form of sex-limitation, it is also possible to determine
whether the same genes are present in both sexes but only ex-
pressed in one sex. This is evaluated by comparing the simi-
larities of opposite-sex DZ twin pairs with same-sex DZ
pairs. Sex-specific genetic influences are suggested when the
similarity of opposite-sex pairs is significantly less than
the similarities of male or female DZ pairs. The difference in
the correlation is attributable to the gender composition of
each zygosity group. When the same and opposite-sex DZ
correlations are similar, gender differences are not indicated.
Only a few studies have investigated sex-limited gene
expression in normal personality. The most notable is Finkel
and McGue’s (1997) study that showed that thesamegenetic
loci influence 11 out of the 14 scales of Multidimensional
Personality Questionnaire (MPQ; Tellegen, 1982) in males
and females. The heritable influences on the remaining threeTABLE 3.6 Model-Fitting Statistics
Canadian Sample German Sample
Model ^2 p RMSEA AIC ^2 p RMSEA AIC
Neuroticism
1 a 199.91 .00 .040 −64.09 216.56 .00 .039 −47.44
2 b 172.11 .00 .036 −79.89 149.82 .07 .019 −102.18
3 c 151.12 .00 .029 −88.88 135.86 .15 .015 −104.14
4 d 144.88 .03 .029 −83.12 131.14 .13 .016 −96.86
5 e 145.12 .03 .030 −82.88 130.40 .14 .014 −97.60
6 f 210.86 .00 .043 −61.14 220.57 .00 .038 −51.43
Note. All models specified additive genetic and nonshared environmental factors unique to each facet. adf=132, one
common additive and one common nonshared environmental factor. bdf=126, one common additive and two common
nonshared environmental factors. cdf=120, two common additive and two common nonshared environmental factors.
ddf=114, two common additive and three common nonshared environmental factors. edf=114, three additive and two
nonshared environmental factors.fdf=136, common pathways model.