 30. Attachment Theory and Personality  659
studying the etiology of individual differ -
ences in attachment.
Some Further Considerations and  Directions 
for Future Research on the Antecedents 
of Attachment  Security
One of the themes we have emphasized in 
this chapter is that there is great value in 
conceptualizing and assessing attachment 
in a contextually sensitive way; that is, the 
way people relate to their parents does not 
wholly overlap with the way they relate to 
their romantic partners. We know that there 
are positive correlations across relational 
domains (which is why the concept of a gen -
eralized attachment style is defensible), but 
there is plenty of room for divergence in the 
ways people relate to different significant 
others in their lives. This within-  person het -
erogeneity raises fascinating possibilities for 
understanding the behavior genetics of adult 
attachment. It is possible, for example, that 
heritable differences between people con -
tribute to the shared variance across rela -
tional domains. If this is the case, it could be 
that the thread that connects relational pat -
terns across domains has, in part, a genetic 
explanation, but that genetics do not explain 
what makes the same person different across 
domains.
Another issue that needs to be clarified 
in the future is age-  related differences in 
genetic and environmental influences. In be -
havior genetics research on attachment and 
interpersonal relationships in childhood, re -
searchers have consistently found relatively 
small estimates of genetic influence and 
sometimes sizable estimates of shared en -
vironmental influences (e.g., O’Connor & 
Croft, 2001; Roisman & Fraley, 2006). The 
measurement of attachment in childhood 
and adulthood is different, so we cannot 
assume that the weaker genetic estimates 
found for children reflects developmental 
processes rather than measurement differ -
ences. But these findings do raise the pos -
sibility that additive genetic factors come to 
play a larger role in explaining attachment 
differences as individuals move from child -
hood to adulthood (see, e.g., Barbaro et al., 
2017, and Fearon, Shmueli-  Goetz, Viding, 
Fonagy, & Plomin, 2014). This kind of pat -
tern has also been observed for measures of cognitive ability, which show consider -
able shared environmental effects in early 
childhood but sizable genetic effects by early 
adulthood (Briley & Tucker-  Drob, 2013). If 
this pattern actually applies to attachment, 
then understanding the relative contribution 
of genetic and environmental factors may 
require a focus on developmental processes 
that have not been fully addressed up to this 
point.
Another issue that requires further consid -
eration is that as individuals transition from 
adolescence to adulthood, they increasingly 
use peers, as opposed to parents, as attach -
ment figures (Fraley & Davis, 1997; Nicker -
son & Nagle, 2005). This opens the door for 
nonfamilial factors to shape attachment dif -
ferences in ways that could diminish to role 
of influences from the family of origin. It is 
not clear, however, what role heritable fac -
tors play in this process. It could be the case, 
as suggested by the mediational process de -
scribed previously, that people select them -
selves into relationships that are compatible 
with the working models they already hold. 
And to the extent that there is genetic vari -
ance underlying these, that variance will be 
reflected in additive genetic contributions 
to individual difference in behavior genetics 
models.
Another possibility is that the role of heri -
table factors will fade across time because, 
although selection processes may be active, 
socialization processes are also active, and 
people’s attachment styles are partial reflec -
tions of experiences that are independent 
of existing representations (Fraley, 2002). 
Those nonshared experiences can accumu -
late to diminish the role of heritable factors 
in explaining differences between people.
We have highlighted these issues in some 
depth for a number of reasons. First, we are 
concerned that researchers will ignore the 
potentially confounding role of genes when 
trying to understand individual differences 
in attachment style. We would like for re -
searchers to appreciate that alternative ex -
planations exist and that considering them 
should make it possible to advance our un -
derstanding of attachment and its develop -
ment.
But more importantly, we are genuinely 
enthusiastic about the work that can be done 
at the interface of behavior genetics and at -