158 II. BIOLOGICAL  FOuNDATIONS
reported an average heritability of .39, with 
a higher level (.47) in twin studies than in 
those that used adoption or family study 
methods (.22). This is because the latter 
are not well-  suited to modeling nonaddi -
tive genetic variance (Plomin, Corley, Caspi, 
Fulker, & DeFries, 1998), which gives rise to 
“emergenetic” traits—  those influenced by a 
configuration, rather than a simple sum, of 
polymorphic genes (Lykken, McGue, Telle -
gen, & Bouchard, 1992). See Krueger and 
Johnson (Chapter 9, this volume) for a fuller 
discussion of this issue.
As stated earlier, the data are particularly 
extensive for N/NE and E/PE, and it is note -
worthy that Vukasovi ć and Bratko (2015) 
had sufficient studies using Eysenck’s mea -
sures (Eysenck, 1964; Eysenck & Eysenck, 
1975), Tellegen’s Multidimensional Person -
ality Questionnaire (MPQ; see Tellegen et 
al., 1998) and the FFM to determine that 
the personality model/measure used was not 
a significant moderator of heritability esti -
mates for either trait. Use of the CPI (Loeh -
lin & Gough, 1990), the Minnesota Multi -
phasic Personality Inventory (MMPI; Beer, 
Arnold, & Loehlin, 1998), and Cloninger’s 
Temperament and Character Inventory 
(Keller, Coventry, Heath, & Martin, 2005) 
yielded virtually identical findings; see also 
Loehlin, McCrae, Costa, and John (1998) 
for a study examining multiple measures in 
the same sample.
The data for DvC also have been quite 
consistent, with the exception of one unusu -
ally low heritability estimate (.18) for the 
CPI Norm-  Favoring vector scale (Loehlin 
& Gough, 1990). However, the data are in -
consistent regarding the extent to which the 
genetic variance is additive versus nonaddi -
tive (see Keller et al., 2005). Nevertheless, it 
is noteworthy that nonadditive dominance 
effects are found much more frequently for 
E/PE than for the other major dimensions 
(Eysenck, 1990; McGue, Bacon, & Lykken, 
1993; but see Loehlin et al., 1998, who also 
found them for N). Finally, it is interesting 
to note that—  paralleling the structure of 
phenotypic personality traits—  the factor 
structure that emerges from genetic studies 
is hierarchical (Krueger & Johnson, Chapter 
9, this volume).
Second, the data overwhelmingly suggest 
that a common rearing environment (i.e., the effects of living together in the same 
household) exerts virtually no effect on per -
sonality (e.g., Beer et al., 1998; Bouchard & 
Loehlin, 2001; Eid, Reimann, Angleitner, 
& Borkenau, 2003; Eysenck, 1990; Finkel 
& McGue, 1997; Luciano, Wainwright, 
Wright, & Martin, 2006; Eysenck, 1990; 
Goldsmith, Buss, & Emery, 1997; Jang et 
al., 2002; Loehlin et al., 1998; Plomin & 
Daniels, 1987; Tellegen et al., 1988). This 
finding was unanticipated and initially 
was met with some skepticism, but it has 
emerged so consistently that it now must be 
acknowledged as a necessary component in 
any model of personality development. In -
deed, virtually the only supportive evidence 
comes from analyses of the E/PE factor: At 
least three studies have reported significant 
shared environment effects for E/PE (Beer et 
al., 1998; Goldsmith et al., 1997; Tellegen et 
al., 1988). Loehlin et al. (1998) also found 
shared environment effects for A and C on 
personality inventories but not trait or ad -
jective rating scales. However, this positive 
evidence must be weighed against a much 
larger number of studies that have found no 
effects due to the common rearing environ -
ment (e.g., Eysenck, 1990; Finkel & McGue, 
1997; Jang et al., 1998). Thus, we largely 
concur with Beer et al.’s (1998) conclusion 
that “shared familial environmental varia -
tion is an unimportant source of individu -
al differences in personality and interests” 
(p. 818).
In contrast, researchers consistently have 
reported substantial effects due to the un -
shared environment (i.e., idiosyncratic en -
vironmental stimuli experienced by a single 
individual but not by his or her biological 
relatives). Given that the shared environment 
has little to no effect, the most commonplace 
finding is that the variance not attribut -
able to genes is almost entirely attributable 
to unique aspects of the environment (see 
Bouchard & Loehlin, 2001; Loehlin et al., 
1998; Plomin & Daniels, 1987; Tellegen et 
al., 1988). However, this almost certainly 
represents a substantial overestimate of 
“true” unshared environmental influence. 
The well-  recognized problem is that the tra -
ditional method of estimating the unshared 
environment confounds (1) true environ -
mental variance, (2) gene–  environment in -
teractions, and (3) measurement error (e.g., 