 9. Behavioral Genetics and Personality  221
the forefront of research in personality ge -
netics.
Evidence from  Adoption  Studies
In addition to twin samples, samples of 
adopted individuals can contribute to un -
derstanding genetic and environmental in -
fluences on personality. Adoption creates 
families in which genetic and environment 
influences can be distinguished. Consider, 
for example, families with both adoptive and 
biological children. All the children in those 
families, both biological and adopted, share 
a home environment, and the biological chil -
dren share genetic influences both with each 
other and with their parents. However, the 
adoptive children share genetic influences 
with neither the biological children nor with 
their common parents, because they are ge -
netically unrelated.
Adoption studies are rarer than twin stud -
ies due to the relatively greater ease of re -
cruiting population-  representative samples 
of twins. In addition, parents who adopt 
children tend to have higher socioeconomic 
status than parents in the population over -
all. This, combined with a tendency for 
people of higher socioeconomic status to 
participate more readily in scientific stud -
ies, makes it particularly difficult to obtain 
adoption samples that adequately represent 
persons of lower socioeconomic status. Per -
haps at least partly because of these factors, 
adoption studies come to slightly differ -
ent conclusions than do twin studies. Like 
twin studies, not only do adoption studies 
generally show little influence of the shared 
environment, but they also yield smaller es -
timates for genetic influence on personality 
and temperament (Loehlin, Horn, & Will -
erman, 1981; Loehlin, Willerman, & Horn, 
1982; Plomin, Coon, Carey, DeFries, & 
Fulker, 1991; Plomin, Corley, Caspi, Fulker, 
& DeFries, 1998; Scarr, Webber, Weinberg, 
& Wittig, 1981).
Beyond differences in population repre -
sentativeness, there are a number of possible 
reasons for differences between estimates of 
genetic influence on personality derived from 
twin and adoption studies (for recent meta- 
analytic evidence, see Vukasovic & Bratko, 
2015). One possibility is that identical twins 
reared together tend to assimilate or imitate each other. This possibility can be evaluated 
by comparing similarity in twins reared to -
gether with similarity in twins reared apart. 
Two major studies of twins reared apart 
reached somewhat different conclusions 
on this issue. A study from Sweden showed 
greater similarity among reared-  together 
MZ twins when compared with reared-  apart 
MZ twins (Pedersen, Plomin, McClearn, & 
Friberg, 1988). This suggests assimilation on 
the part of the twins reared together, though 
it could also indicate shared environmental 
influences or gene-  shared environmental 
correlation that affects MZ twins more than 
DZ twins due to their greater physical resem -
blance. However, another study showed the 
resemblance of MZ twins reared apart and 
together to be similar for personality traits 
(Tellegen et al., 1988). This could indicate 
that some MZ twins reared together assimi -
late, while others work to differentiate from 
each other, and/or that environmental op -
portunities to express and respond to geneti -
cally influenced behaviors are strong enough 
to match whatever assimilation or shared en -
vironmental influences tend to make reared- 
together MZ twins more similar. In addition, 
conditions could vary among social groups, 
countries, and cultures, so that no general 
“rule” necessarily applies.
Another possibility that receives more 
consistent support involves nonadditive ge -
netic influences. Genetic influences can be 
additive, meaning that the multiple genes 
that contribute to observed variation in a 
characteristic are fungible, each indepen -
dently contributing to continuous variation 
in an observable characteristic, or pheno -
type. Genetic influences can also be nonad -
ditive, meaning that specific combinations 
of relevant individual genetic markers are 
important in understanding the resulting 
trait. The most commonly used example 
of nonadditive genetic influences is simple 
Mendelian dominance, in which the re -
sulting trait in the heterozygous type (non -
matching pair of genetic markers) resembles 
one but not the other of the homozygous 
types (matching pair of genetic markers). 
MZ twins share both additive and nonad -
ditive genetic influences completely, because 
they share the same genotype. Because first- 
degree relatives, such as siblings, DZ twins, 
and parents and offspring, share on average 