332 III. DEvELOPMENT
behavioral, cognitive, and affective changes 
associated with a life transition occur in an 
incremental way, then we would expect per -
sonality trait changes to unfold over longer 
periods following the transitional experi -
ence. Consequently, longitudinal studies 
that begin shortly before a transition and 
include only one or few follow-  ups after the 
transition may mistake preevent changes 
for stable preexisting differences and/or 
fail to detect temporally delayed effects. To 
detect possible pre- and postevent changes 
and to locate critical periods of change sur -
rounding a transition, prospective studies 
are needed that measure personality mul -
tiple times before, during, and after a major 
life transition.
Third, because experimental designs are 
not feasible, researchers must rely on other 
research designs that use additional pieces of 
information to test personality trait change 
in response to life transitions, while ruling 
out potential confounds. Recently, research -
ers have begun to include matched control 
groups that did not experience the life tran -
sition of interest (e.g., Wagner et al., 2015; 
Van Scheppingen et al., 2016). These de -
signs allow researchers to account for the 
confounding influence of measured  control 
variables. A particularly well-  matched con -
trol group that would allow researchers 
to also control for unmeasured  influences 
would consist of monozygotic twin siblings. 
Monozygotic twins share both a common 
genotype and a similar rearing environment. 
Hence, longitudinal co-twin control stud -
ies on monozygotic twins who are discor -
dant on exposure to certain life transitions 
would combine the advantages of observa -
tional studies and experimental designs, 
because monozygotic twins are matched on 
a multitude of known and unknown poten -
tial confounding factors, including their ge -
netic background (McGue, Osler, & Chris -
tensen, 2010). Strong support for the effect 
of life transitions on personality trait change 
would thus be provided by a prospective co-
twin control study showing that monozygot -
ic twins who differ in the timing of specific 
life transitions also differ in their personal -
ity trajectories.
Fourth, in most studies, personality traits 
were assessed with self-  reports, which may 
be prone to biases that distort conclusions 
regarding the impact of life events on person -ality trait change. Multimethod approaches 
using informant reports, behavioral data, 
or other kinds of assessments would enrich 
research on life events and personality trait 
change. Discontinuities in the patterns ob -
served across assessment methods might 
also be illuminating (Bornstein, 2012). Also, 
as outlined previously, experience-  sampling 
or event-  sampling designs have rarely been 
used to study personality trait change dur -
ing the experience of major life transitions 
(Wrzus & Roberts, 2017). These approach -
es, while often still relying on self-  report, 
have the advantage of illuminating the un -
derlying short-term processes of personality 
trait change as they occur during the expe -
rience of life transitions (Fleeson & Jolley, 
2006; Wrzus & Roberts, 2017).
Fifth, as mentioned earlier, the vast major -
ity of studies has examined the impact of life 
transitions on rank-order change or mean-
level change in personality traits. These two 
types of change describe changes in indi -
vidual personality traits at the population 
level. However, changes at the population 
level may not always reflect changes at the 
individual level. In particular, more research 
is needed to understand when and how (1) 
individuals deviate from the general popula -
tion trends (i.e., individual-  level change) and 
(2) how the configuration of traits within an 
individual changes (i.e., ipsative personality 
change) in response to a specific life tran -
sition (Robins, Fraley, Roberts, & Trzesn -
iewski, 2001).
Sixth, large sample sizes are needed to de -
tect the presumably small effects of life tran -
sitions on personality trait change. Many 
previous studies compared relatively small 
subsamples of participants who had or had 
not undergone a particular life transition. 
Until replicated in larger samples, the re -
sults of these studies should be interpreted 
with caution. Moreover, virtually all previ -
ous studies on personality trait change have 
examined samples from the United States or 
other Western, educated, industrialized, rich, 
and democratic (WEIRD; Henrich, Heine, 
& Norenzayan, 2010) nations. A systematic 
longitudinal examination of personality trait 
change and life transitions across a large and 
diverse set of cultures has yet to be undertak -
en. Such an analysis is needed to test whether 
the reported effects of life transitions on per -
sonality traits represent cross-  cultural uni -