76 Genetic Basis of Personality Structure
TABLE 3.8 Parameter and Standard Error Estimates Produced by the Best-Fitting Sex-Limitation Model
Parameter N E O A C
Canada
hf .86±.03 .84 ±.03 .80 ±.02 .85 ±.03 .86 ±.03
ef .82±.02 .84 ±.02 .88 ±.01 .83 ±.02 .82 ±.02
hm .80±.04 .84 ±.04 .80 ±.02 .89 ±.03 —
em .88±.03 .84 ±.03 .88 ±.01 .79 ±.03 .85 ±.03
h′m ————.83±.04
Germany
hf .84±.02 .87 ±.02 .80 ±.02 .84 ±.02 .82 ±.02
ef .84±.01 .81 ±.01 .88 ±.01 .84 ±.01 .86 ±.01
hm .84±.04 .87 ±.04 .81 ±.04 .87 ±.03 —
em .84±.03 .81 ±.03 .87 ±.03 .81 ±.03 .81 ±.03
h′m ————.87±.03
Note. N=Neuroticism; E =Extraversion; O =Openness to Experience; A=Agreeableness; C =Conscientiousness; hf, ef,
hm, em=additive genetic and nonshared environmental effects common to males and females; h′m=male-specific additive
genetic effects.thoroughly. As such, the present results should be considered
tentative until replicated on a larger sample using full-scale
versions, as well as other measures of personality.
These analyses suggest that although most personality
traits are influenced by the same genes in both genders (the
implication being, e.g., DRD4 influences novelty seeking in
both men and women), this is not true for all traits. The
previous section suggested several explanations, but it is also
possible that at the molecular level, different genes (or yet-to-
be-discovered polymorphisms) differentially influence per-
sonality across genders. If this is the case, the genetic and
environmental architecture of some scales may differ by gen-
der. This could be evaluated by fitting independent and com-
mon pathways models to data from sister pairs and brother
pairs separately and constraining the models (in form and
magnitude) to be equal across gender groups. The sex-limita-
tion model described previously that uses data from brother-
sister pairs to test for gender-specific effects can be expanded
to the multivariate case to further explore gender differences
in personality. As far as we are aware, few multivariate ge-
netic analyses of gender differences have been conducted,
probably because many studies have limited data collection
to sister pairs or have difficulty obtaining data from brother
pairs (Lykken, McGue, & Tellegen, 1987).
ENVIRONMENTAL EFFECTS
Although our primary concern is with the genetic basis for
personality structure, any discussion of genetic influence
would be incomplete without reference to environmental fac-
tors. Twin studies consistently show that about 50% of the
variance in personality traits is explained by environmental
factors and that most of this is accounted for by nonspecific
influences; common environmental influences do not appear
to contribute to personality variation (Plomin & Daniels,
1987). This etiological model derived from twin studies is
confirmed by a large-scale study of Neuroticism by Lake,
Eaves, Maes, Heath, and Martin (2000) that showed that in-
dividual differences in neuroticism were not transmitted from
parent to offspring via the environment but rather by genetic
factors. The size and unique features of their data set (45,880
twin pairs andtheir relatives on two continents) allowed
them to test models of genetic transmission as well as gene-
environment correlations. The results suggest that the envi-
ronment exerts a contemporaneous influence on individual
differences in neuroticism. That is, its effects are located in
the current environment as opposed to being preset like ge-
netic factors that are passed to individuals from their parents.
Although nonshared environmental factors are important,
the nature of these variables and the way they affect person-
ality remain unclear. Despite considerable research effort
(e.g., Hetherington, Reiss, & Plomin, 1994; Turkheimer &
Waldron, 2000) using a variety of methods (Baker & Daniels,
1990; Hetherington et al., 1994; Reiss et al., 1994; Vernon,
Lee, Harris, & Jang, 1996) the results have been uniformly
disappointing: Few nonshared influences on personality have
been identified (Turkheimer & Waldron, 2000). Most studies
have, however, investigated the effects of the nonshared en-
vironment on the single variables; few studies have examined
the effects of the nonshared environment on trait covariance.
The study by McCrae et al. (in press) and the illustrative mul-
tivariate genetic analyses of the NEO-PI-R and DAPP pre-
sented earlier suggest that the nonshared environmental
factors have an influence on personality structure different
from that of genetic factors. They do not appear to contribute
to trait substantially to the trait covariation described by trait
taxonomies.