222 II. BIOLOGICAL  FOuNDATIONS
only 50% of their genetic variants, resem -
blance can be greater for additive than non -
additive genetic influences, depending on 
the importance of dominant genetic markers 
to the trait, and their frequencies in the pop -
ulation relative to that of the recessive mark -
ers. As a result, adoption studies that involve 
first-  degree relatives, such as siblings or par -
ents and their offspring (but not MZ twins) 
can indicate less overall genetic influence on 
personality than twin studies involving MZ 
twins. Specific examples of single dominant 
genetic markers that exert important in -
fluences on personality traits are presently 
unknown, and studies so far suggest there 
are unlikely to be many single markers with 
large influences. There are, however, many 
other forms of nonadditive gene actions, and 
some of these could be important contribu -
tors to personality variation.
Loehlin, Neiderhiser, and Reiss (2003) 
provided a very informative analysis of non -
additive genetic effects on personality varia -
tion derived from data collected as part of 
the Nonshared Environment in Adolescent 
Development (NEAD) study (Reiss, Neider -
hiser, Hetherington, & Plomin, 2000). The 
NEAD study was unique in including not 
just MZ and DZ twin pairs, but also full- 
sibling pairs in intact families, full-  sibling 
pairs in remarried families, half-  siblings 
in remarried families, and genetically un -
related siblings in remarried families. The 
existence of these non-twin sibling pairs 
allowed Loehlin and colleagues (2003) to 
examine the impact of MZ pairs on herita -
bility estimates for a series of dimensions of 
both adjustment and maladjustment, akin 
to personality constructs. In analyses ex -
cluding the MZ pairs, smaller estimates of 
genetic influence were obtained, suggesting 
the importance of nonadditive genetic influ -
ences in understanding personality (cf. Lyk -
ken, Bouchard, McGue, & Tellegen, 1992) 
and/or some of the alternative possibilities 
we noted earlier. Complexity at the genom -
ic level (cf. Power & Pluess, 2015), as well 
as these other possibilities involving gene– 
environment transactions, may well be part 
of the reason that linking specific alleles 
with behavior has proven very challenging, 
as we describe in greater detail below.
In summary, we began this section by pos -
tulating that personality results from both genetic and environmental influences. This 
postulate is readily supported by evidence 
stemming from both twin and adoption 
designs, and including diverse ways of as -
sessing personality. Nevertheless, adoption 
designs tend to show weaker genetic effects 
than do twin designs. Various explanations 
for this difference are plausible, and reso -
lution of these possibilities will likely stem 
from combining twin and adoption evidence 
with molecular evidence. For example, the 
unique combinations of alleles shared with -
in MZ pairs may explain the stronger genet -
ic influences observed in twin studies, and 
it may ultimately be possible to study this 
phenomenon at the molecular level.
The Hierarchical Structure of  Personality 
Based on  Genetic Covariation Closely 
Resembles the Structure Based on Observed, 
Phenotypic  Covariation
If we take the findings of numerous behav -
ioral genetics studies of personality varia -
tion seriously, the nature–  nurture debate is 
to some extent over, at least if the debate is 
framed as dominance of one or the other; 
that is, every individual-  differences charac -
teristic that could be measured is probably at 
least somewhat heritable, but environmen -
tal influences are also typically as great as 
the influences of genes (Turkheimer, 2000). 
Does this mean that behavioral genetic stud -
ies of personality have outlived their useful -
ness?
The answer to this question is “no,” be -
cause the study of variation  is only one aspect 
of what can be learned by parsing the con -
tributions of nature and nurture to person -
ality. For example, much inquiry in person -
ality psychology is focused not on variation 
but on covariation. An especially important 
topic has been the structure of personality, 
or the way in which individual differences 
in personality are organized. When behav -
ioral genetics techniques are used to address 
personality structure, investigators estimate 
not only genetic and environmental influ -
ences on personality variation but also the 
extent to which covariation in pairs of traits 
reflects genetic and environmental sources. 
For example, the extent to which genetic ef -
fects are common to pairs of traits can be 
indexed by genetic correlations, which are 