 15. Personality Development in Adulthood and Later Life  337
emphasizing empirical advances that are 
change- or trajectory-  oriented. With respect 
to the former, we describe recent efforts to 
predict trajectories, focusing on naturally 
occurring factors (life or health events, so -
cial role changes), as another chapter in this 
handbook addresses interventional or vo -
litional factors that bring about change in 
personality (Chapter 38, this volume; see 
also Roberts et al., 2017). Then, we flip the 
outcome variable and describe efforts that 
used personality change to predict other 
outcomes later in life (Allemand, Job, & 
Mroczek, 2019; Steiger, Allemand, Robins, 
& Fend, 2013; Mroczek & Spiro, 2007; Al -
lemand & Flückiger, 2017). We end by de -
scribing several theoretical advances in adult 
personality development since publication of 
the previous handbook.
It may seem strange that we relegate theo -
ry to the end, when most authors would lead 
off with it. However, we feel the empirical 
work in this area is of such high quality that 
it makes sense to place it first. Leading off 
with data and considering theory afterward 
also permits a better sense of what theoreti -
cal notions align most closely with current 
empirical knowledge. We are particularly 
sensitive to the latter, for the recent replica -
tion crisis in psychology (Simmons, Nelson, 
& Simonsohn, 2018) has highlighted the 
danger of theory getting too far out ahead 
of data. When everyone has a theory and 
most of those theories are based on one or 
two underpowered studies, and when addi -
tional theory is built on top of this sparse 
edifice, a shaky house of cards is inevitably 
the result. This dismal state of affairs luckily 
does not characterize the area of adult per -
sonality development, in large part due to a 
long-term emphasis on longitudinal studies 
with large sample sizes and an appreciation 
of appropriate and sophisticated modeling 
techniques. Theory in this area tends to be 
well-  grounded in data. It is still useful, how -
ever, to be on guard for theoretical overstep -
ping. Constant comparison of the best avail -
able data with the most current theory, with 
frequent expunging of theory that does not 
match empirical knowledge, is necessary for 
keeping the area of personality development 
rigorous, robust, and replicable.
That said, trajectory models are not as 
easily carried out as correlation coefficients between two measurement occasions (as -
sessing rank-order stability) or comparisons 
of means over time (assessing mean-level sta -
bility). Data and modeling requirements are 
more considerable when estimating trajecto -
ries. This has made such models less wide -
spread and presents challenges with regard 
to replicability of findings. In the next sec -
tion, we address these methodological and 
replicability issues.
Three Levels of  Trajectories
To estimate individual trajectories, multiple 
waves of data are needed, along with use of 
an appropriate modeling technique, such as 
latent growth curves or the multilevel model 
for change. Once multiple waves are avail -
able for a given personality characteristic, 
it is easy to imagine fitting a line or curve 
to the various time points, especially for a 
given person. Person A has a certain pattern, 
and his or her own intraindividual curve; 
person B has a different pattern, and thus a 
differently shaped curve. Each individual in 
a sample or population has his or her own 
trajectory for that personality characteristic. 
This leads to the first and bottom level of 
our own three-tier framework, that of the 
individual trajectory. For any personality 
characteristic, everyone has his or her own 
unique trajectory defined by level (intercept), 
rate of change (linear slope), and curvature 
(quadratic slope, cubic slope, and beyond). 
For a given personality characteristic, pic -
ture 7.7 billion of these trajectories for each 
person alive in the world at present. Imagin -
ing this gargantuan plot of unique curves al -
lows us to grasp another powerful phenom -
enon in personality development research, 
one that is also a cornerstone of lifespan 
developmental theory (Baltes, 1987). This is 
the concept of individual differences in in -
traindividual trajectories. Evoking Murray 
and Kluckhohn, Oliver’s trajectory is like no 
other person’s trajectory.
Imagine a second level, representing a 
grouping of trajectories. The individual- 
person-  level trajectories can be averaged into 
a group-level trajectory. Picture trajectories 
for particular subgroups, such as extraver -
sion trajectories of women, neuroticism tra -
jectories of adolescents, or conscientiousness 