658 vI. SELF‑ AND S OCIAL P ROCESSES
tions of attachment theory (e.g., Barbaro, 
Boutwell, Barnes, & Shackelford, 2017). On 
the surface, it might seem that, if some por -
tion of individual differences in attachment 
can be attributed to heritable differences be -
tween people, then either a careful exami -
nation of socialization processes (a) is not 
needed or (b) is irrelevant to understanding 
how people become who they are.
We think modern attachment theory is 
an interesting example of how a theory can 
be uncomfortably pulled by the tensions 
between behavior genetic and socialization 
models, while being misrepresented by both 
extremes. Our reading of Bowlby’s original 
theory is that there is room for both hered -
ity and socialization in personality develop -
ment. Bowlby believed that there may be 
heritable differences between children that 
are related to the kinds of caregiving they 
receive and the experiences they have (Bowl -
by, 1973). But, importantly, he believed 
that those interpersonal interactions played 
a causal role in shaping and sustaining in -
dividual differences. In modern parlance, 
Bowlby believed that interpersonal experi -
ences may mediate  the relationship between 
genetic differences and attachment-  related 
outcomes (see Briley et al., 2019, for a de -
tailed discussion of how developmental and 
genetic models can be integrated to lead to 
new ways of understanding development).
We highlight this mediation hypothesis 
for two reasons. First, attachment theory 
is sometimes portrayed as disregarding the 
potential role of genetic factors in shaping 
attachment- related outcomes (e.g., Sher -
lock & Zietsch, 2018; Yarkoni, 2016). And 
although the majority of research on the 
etiology of individual differences in attach -
ment does, in fact, ignore heritable factors, 
we believe that this neglect is not due to the 
belief that genetics are irrelevant; it is due 
to the assumption that genetic influences are 
more distal in the causal chain than inter -
personal ones. Stated differently, the work -
ing assumption is that if heritable factors 
play a role in shaping relationship dynamics, 
those dynamics are primarily responsible for 
the working models people construct about 
their attachment relationships. Moreover, 
because some of the developmental research 
on attachment is motivated by concerns for 
social policy and child welfare, empirical ef -fort has focused on identifying environmen -
tal factors that can be targeted in interven -
tions (e.g., parental responsiveness training, 
creating safe environments).
Second, scholars sometimes assume that 
if there is evidence from twin designs of 
heritable variation in an outcome, then in -
terpersonal factors (e.g., parenting, the re -
sponsiveness and availability of attachment 
figures) are not relevant for understanding 
that outcome. This conclusion implicitly as -
sumes that genetic variation creates individ -
ual differences in people’s experiences and 
in their attachment styles, and  that there is 
no causal path between experience and at -
tachment style.
In our view, empirical research is con -
sistent with the idea that there are causal 
relations between people’s interpersonal 
experiences and their attachment styles. 
When people are experimentally placed in 
situations in which they are made to feel 
vulnerable, neglected, or misunderstood by 
their partners, they tend to experience in -
creases in insecurity (e.g., Carnelley, Otway, 
& Rowe, 2016). However, most of this re -
search is based on short-term laboratory ex -
periments. As a result, it is not clear whether 
the changes in attachment that have been 
observed are merely short-term deviations 
from people’s attachment patterns or more 
enduring changes. Thus, an important di -
rection for future research is to understand 
what kinds of factors influence attachment 
styles and whether those influences are tran -
sient or enduring (see Fraley et al., 2013).
In summary, although attachment re -
searchers have generally overlooked herita -
ble sources of variation in studying the ori -
gins of attachment styles, we believe this is 
a valuable area for future research. Indeed, 
investigating the genetic and environmental 
factors that underlie adult attachment pat -
terns represents a more inclusive and prom -
ising approach than focusing on one etiolog -
ical class of explanations alone. Moreover, 
contrary to some assumptions, we do not 
believe that the presence of heritable varia -
tion in twin studies implies that parenting 
or interpersonal factors are poor candidates 
for understanding attachment development. 
However, the fact that these sources of vari -
ance can be confounded does imply that 
sophisticated designs will be necessary for 