 30. Attachment Theory and Personality  655
these experiences translate into outcomes 
later in adulthood. There is an enormous 
amount of interest in these issues in con -
temporary psychology, because many of the 
infant and child participants in longitudinal 
studies that were inspired by Bowlby and 
Ainsworth’s ideas decades ago have come 
of age in recent years (e.g., Chopik, Moors, 
& Edelstein, 2014; Fraley, Roisman, Booth- 
LaForce, Owen, & Holland, 2013; Salo, 
Jokela, Lehtimäki, Keltikangas-  Järvinen, 
2011; Zayas, Mischel, Shoda, & Aber, 
2011). As a result, we have learned a great 
deal about the development of attachment 
and attachment-  relevant outcomes since the 
previous edition of this chapter was written.
In one of the foundational papers in this 
new generation of longitudinal projects, 
Simpson, Collins, Tran, and Haydon (2007) 
analyzed data from 78 individuals who had 
been followed from infancy to adulthood 
(ages 20–23 years). Simpson and his col -
leagues found that early attachment secu -
rity, assessed in the Strange Situation, was 
related to social competence in childhood, 
which in turn was related to friendship qual -
ity in adolescence, and still later, predicted 
positive emotional experiences in close rela -
tionships at age 23. Based on these findings, 
Simpson and his colleagues suggested that 
early attachment experiences have the po -
tential to indirectly shape later interpersonal 
outcomes. Importantly, the process they de -
scribed is not deterministic (e.g., some pro -
portion of secure children enter into poorly 
functioning friendships). But, when stepping 
back and viewing development through a 
longitudinal lens, it is possible to see how 
early interpersonal processes trickle down 
into other domains of life, pushing people 
along some pathways rather than others.
In another important study, Dinero, 
Conger, Shaver, Widaman, and Larsen- 
Rife (2008) studied the quality of observed 
interactions between approximately 250 
adolescents and their parents over a peri -
od of 10 years. They found that the qual -
ity of parent–  adolescent interactions at age 
15 predicted attachment security at age 25 
after controlling for assessments of security 
at age 15. This research was important for at 
least two reasons. First, the work took into 
account the autoregressive nature of attach -
ment security, showing that, despite the “in -trinsic” stability of attachment, interperson -
al experiences at one point in time have the 
potential to predict what people will be like 
at another point in time. Second, the work 
emphasized the role of adolescence rather 
than early childhood exclusively. We elabo -
rate on this point in subsequent sections, but 
for now we wish to make clear that attach -
ment theory emphasizes the role of attach -
ment experiences across  development and 
not merely those experiences that take place 
in early childhood.
In another intensive investigation, Fraley 
and his colleagues (2013) analyzed data 
from the National Institute of Child Health 
and Human Development (NICHD) Study 
of Early Child Care and Youth Develop -
ment. The children in this sample were origi -
nally assessed when they were 1 month of 
age. Importantly, they have been reassessed 
multiple times, with in-depth assessments of 
the quality of the caregiving environment 
(i.e., observed maternal sensitivity, maternal 
depression, father absence) and the quality 
of friendship relationships. At age 18, these 
children completed self-  report measures of 
adult attachment style. Children who were 
relatively secure at age 18 were more likely 
to have had a developmental history char -
acterized by increasingly high-  quality care -
giving and friendship relationships. For ex -
ample, at age 18, those who were relatively 
secure were likely to have mothers who 
showed increases in sensitivity over time and 
decreases in depression, increases in social 
competence, and ever-  improving friendship 
relationships.
Taken together, these studies suggest that 
one reason some people are more secure 
than others is that they have experienced a 
history of warm and responsive interactions 
with others—  both parents and peers. Al -
though such studies do not demonstrate that 
these experiences have a causal influence on 
the development of adult attachment styles 
(we discuss alternative explanations below), 
they help to fill a key gap that has been miss -
ing in the attachment literature.
Differentiation and  Its Implications 
for Understanding Attachment  Development
One of the challenges for future research is 
to understand why the same person can be 