 15. Personality Development in Adulthood and Later Life  341
wein (2012) documented long-  lasting (nega -
tive) changes in agreeableness after military 
training, arguably a unique form of work 
experience. Greater investment in career 
and work also seems to be associated with 
personality changes, particularly growth in 
conscientiousness (Hudson, Roberts & Lo -
di-Smith, 2012).
A third line of research has focused on life 
events and life transitions as predictors of 
subsequent personality change. These have 
identified a host of factors related to person -
ality change, ranging from cultural activity 
(predicting openness; Schwaba, Luhmann, 
Denissen, Chung, & Bleidorn, 2018) to 
positive and negative life events (Ludtke et 
al., 2011), to transitions of various types, 
such as high school graduation (Bleidorn, 
2012), retirement (Löeckenhoff, Terracia -
no, & Costa, 2009; Schwaba & Bleidorn, 
2019; Specht et al., 2011), and becoming 
a parent (Jokela, Kivimäki, Elovainio, & 
Keltikangas-  Järvinen, 2009; Specht et al., 
2011).
However, each of these three lines of re -
search on predictors of personality change 
is replete with null findings (Bleidorn, Hop -
wood, & Lucas, 2018). For example, Mroc -
zek and Spiro (2003), in contrast to Schwaba 
and Bleidorn (2019), did not find retirement 
to be associated with trait change. Specht et 
al. (2011) examined a variety of life events, 
finding only limited effects on trait change, 
with conscientiousness having the strongest 
associations with life events.
There certainly appears to be more than 
reasonable evidence for various external fac -
tors driving personality change, with rela -
tionships perhaps exerting the strongest ef -
fects. Yet there are reasons to believe that 
the picture is far more complicated than 
these findings suggest. We point to three 
bodies of literature.
First, longitudinal twin designs, a small 
number of studies have offered evidence that 
genetic factors are a key driver of individual 
differences in personality change (Bleidorn 
et al., 2009; Blonigen, Hicks, Krueger, Pat -
rick, & Iacano, 2006; McGue, Bacon, & 
Lykken, 1993), and that these genetic effects 
are often larger than environmental effects 
(Kandler, Bleidorn, Riemann, Angleitner, 
& Spinath, 2012). The extent to which we 
experience trait change may have its source within us, in our inherited biology, and only 
partly sourced to our relationships with 
other people or the external events that pep -
per our lives.
Second, a handful of studies has dem -
onstrated transactional or bidirectional re -
lationships between personality traits and 
many other kinds of variables, some of 
which are regularly considered outcomes or 
endpoints. Jackson (2011) discovered that 
conscientiousness and academic engagement 
reinforce each other in a transactional man -
ner, such that increases in conscientious -
ness lead to increases in academic behaviors 
(studying, going to class), which in turn lead 
to further increases in conscientiousness, 
which lead to even further increases in aca -
demic behavior. Essentially, a bidirectional 
relationship operated, making the real de -
terminant of personality change much less 
clear—or academic behavior change for that 
matter. Which was the predictor, and which 
was the outcome? The picture is blurred, 
calling into question our traditional beliefs 
of what is causal and what is caused, and 
what is internal to the person and what is 
external. Lest we take this line of reasoning 
toward argumentum ad absurdum,  there 
are obviously some outcomes that are true 
endpoints, such as divorce, disease onset, 
and mortality. However, many variables re -
lated to personality are ones that continu -
ally change over the lifespan or segments of 
the lifespan: health status, relationship and 
work success, cognitive functioning, hap -
piness and life satisfaction, and so forth. 
These variables comprise the most likely 
candidates for entry into bidirectional (or 
tridirectional, or multidirectional) feedback 
loops with personality, thereby making 
opaque the predictor–  outcome association.
Since Jackson (2011), other research -
ers have provided evidence consistent with 
personality-  related bidirectionality in the 
domains of work experiences (Le, Donnel -
lan, & Conger, 2014), living arrangements 
(Jonkman, Thoemmes, Lüdtke, & Traut -
wein, 2014), relationships (Mund & Neyer, 
2014; Robins et al., 2002) and life events 
(Denissen, Luhmann, Chung, & Bleidorn, 
2019; Kandler, Zimmermann, & McAdams, 
2014). Additional work has shown that even 
changes among traits (as well as cognitive 
ability) can be subject to bidirectional pro -