The Etiological Basis of Covariance 63 63self-directedness as assessed using the Temperament and
Character Inventory and the 5-HTTLPR allele (Hamer,
Greenberg, Sabol, & Murphy, 1999).
To date, a self-report measure of personality that has no
genetic influence has not been identified (Plomin & Caspi,
1998). The qualification should be added that heritability stud-
ies have relied largely on self-report measures—alternative
methods of assessment may yield different results. However,
this was not the case with the few studies using other methods
(Heath, Neale, Kessler, Eaves, & Kendler, 1992; Riemann,
Angleitner, & Strelau, 1997). Riemann and colleagues (1997),
for example, reported a twin study conducted in Germany and
Poland that compared assessments of the five factors using
self-report questionnaires with peer ratings. Estimates of her-
itability based on self-report were similar to those reported by
other studies. The peer ratings also showed evidence of heri-
tability, although estimates were lower than those obtained
from self-reports. Multivariate genetic analyses showed that
the same genetic factors contributed to self-report and peer
ratings. These results suggest that findings of a heritable com-
ponent to all self-report measures are likely to generalize to
other methods of measurement.
Evidence of heritability alone, however, is not sufficient to
justify the use of behavioral genetic criteria to clarify trait
structure. It is possible that environmental factors that ac-
count for about 50% of the variance have a substantial effect
on trait covariation. If this were the case, the finding that
traits are genetically related would be of less value in clarify-
ing personality structure. The evidence, however, suggests
that the phenotypic structure of traits closely parallels the un-
derlying genetic architecture (Livesley, Jang, & Vernon,
1998; Loehlin, 1987)—a point that is discussed in detail later
in this chapter.
It should be noted, however, that information about heri-
tability merely explains the variance in a single trait as op-
posed to the covariance between traits. Such information has
limited value in explicating personality structure. As Turk-
heimer (1998) argued, all individual differences in behavior
are heritable and “... the very ubiquity of these findings
make them a poor basis for reformulating scientists’ concep-
tions of human behavior” (p. 782). Nevertheless, information
on heritability forms the foundation for understanding of the
etiology of personality. The major contribution of behavior
genetics to understanding personality structure, however,
comes from multivariate genetic analyses that elucidate the
genetic structure underlying multiple traits (Carey & DiLalla,
1994). Multivariate analyses extend univariate analysis of the
genetic and environmental influences on a trait to evaluate
genetic and environmental components of the covariation be-
tween two or more traits (DeFries & Fulker, 1986). It is this
extension that promises to contribute to personality theory by
explicating the etiological basis for trait covariance by evalu-
ating the degree to which different traits are influenced by the
same genetic and environmental factors. This issue is central
to resolving some of the problems of personality description
and structure.THE ETIOLOGICAL BASIS OF COVARIANCEThe phenotypic covariation between two traits may be due to
pleiotropy—that is, the degree to which the traits share a
common genetic influence, environmental effects common to
both traits, or both. The degree to which two variables have
genetic and environmental effects in common is indexed by
genetic(rG)and environmental correlation coefficients (rE).
These statistics are interpreted as any other correlation coef-
ficient and they may be subjected to other statistical proce-
dures such as factor analysis (Crawford & DeFries, 1978).
Genetic and environmental correlation coefficients are read-
ily estimated from data obtained from monozygotic (MZ)
and dizygotic (DZ) twin pairs.
The calculation of the genetic correlation is similar to that
used to estimate the heritability of a single variable. A higher
within-pair correlation for MZ twins than for DZ twins sug-
gests the presence of genetic influences because the greater
similarity is directly attributable to the twofold increase in
genetic similarity in MZ versus DZ twins. In the multivariate
case, a common genetic influence is suggested when the MZ
cross-correlation (the correlation between one twin’s score
on one of the variables and the other twin’s score on the other
variable) exceeds the DZ cross-correlation.
The phenotypic correlation (rp) between two variables
(traits),xandy, is expressed by the following equation:rp=(hx·hy·rg)+(ex·ey·re) (3.1)where the observed or phenotypic correlation, (rp),is the
sum of the extent to which the same genetic (rg)and/or envi-
ronmental factors (re)influence each variable, weighted by
the overall influence of genetic and environmental causes on
each variable (hx,hy,ex,ey,respectively). The terms hand
eare the square roots of heritability and environmental effect
(h^2 ande^2 ) for variables xandy,respectively.
It should be noted that a genetic correlation describes
statistical pleiotropism—that is, the extent to which allelic
effects on trait predict allelic effects on the other trait. As
Carey (1987) pointed out, statistical pleiotropism is not to be
confused withbiological pleiotropismin which two variables