 11. Personality Development across the Life Course 271
in their lives that help maintain their current 
trait levels (see also Nye & Roberts, 2019). 
Either because niche picking results in fewer 
expectations for change or the positive af -
fect that sometimes comes with person– 
environment fit, the assumption is that this 
type of person–  environment transaction 
should lead to greater personality trait con -
sistency.
To our knowledge, there have been only 
two studies testing the niche-  picking prin -
ciple. One study took place at a highly 
competitive university, where students who 
were both high in extraversion and low in 
agreeableness fit into that environment bet -
ter than students without that combination 
of personality traits. This better fit resulted 
in more continuity and less change in these 
students’ personality traits (Roberts & Rob -
ins, 2004). This finding was partially repli -
cated by another study at a different highly 
competitive university (Harms, Roberts, & 
Winter, 2006). But, similar to the several of 
the principles we describe below, there is in -
sufficient evidence to make any firm conclu -
sions about the veracity of the niche-  picking 
principle. The niche-  picking principle may 
serve as a complement to the corresponsive 
principle to help clarify the latter and to pro -
vide motivation to test the former.
The Plasticity  Principle
According to the plasticity principle, per -
sonality traits may change at any point in 
a person’s life. Much evidence exists for 
mean-level changes in personality traits 
throughout the life course. Young adulthood 
is the primary age period when mean-level 
change in personality occurs, mostly for the 
better (i.e., increases in conscientiousness, 
decreases in neuroticism; Donnellan et al., 
2007; Johnson et al., 2007; Josefsson et al., 
2013; Vecchione et al., 2012). But there also 
is evidence for mean-level change in both 
middle adulthood (Allemand, Gomez, & 
Jackson, 2010) and older adulthood (Kand -
ler, Kor  nadt, Hagemeyer, & Neyer, 2015; 
Wortman, Lucas, & Donnellan, 2012). 
Sometimes personality trait changes in older 
adulthood are decreases in positive traits, re -
flecting approaching senescence.
The plasticity principle is supported by 
findings on individual differences in person -ality trait change described earlier. Previous 
work has uncovered associations between 
personality traits and life experiences, in -
cluding experiences in relationships (Lehnart 
et al., 2010), stressful life events (Jeronimus 
et al., 2014; Laceulle et al., 2012), and work 
experiences (Le et al., 2014) in adolescence 
and young adulthood. There is similar evi -
dence for individual differences in personal -
ity trait change in middle age (van Aken et 
al., 2006) and old age (Mõttus et al., 2012). 
For instance, in a study of older adults (ages 
60–90), changes in perceived social support 
were related to changes in conscientiousness 
(Hill et al., 2014). Similarly, Lodi-Smith 
and Roberts (2012) reported that changes 
in social engagement in old age were associ -
ated with changes in conscientiousness and 
agreeableness. While some traits may be less 
plastic in middle and old age (Allemand et 
al., 2010), most findings provide support for 
the plasticity principle across the life course.
Finally, with recent research showing the 
effect of therapy on personality trait change 
(e.g., Roberts, Luo, et al., 2017), the plas -
ticity principle is strongly supported by the 
empirical evidence. Most importantly, in the 
meta-  analysis of the relation between therapy 
experiences and personality trait change, the 
findings were not moderated by age. Thus, 
young, middle aged, and older individuals 
were equally likely to show personality trait 
change as a result of therapy. It should be 
noted that evidence for the plasticity of per -
sonality trait change does not mean that the 
preponderance of people transform them -
selves on a day-to-day, month-to-month, or 
year-to-year basis. When examined in detail 
and within a person, the average number of 
traits that show robust changes in any given 
period of the life course is one in five (Rob -
erts et al., 2001). The capacity for change, 
and the existence of evidence for change, 
does not necessarily mean all people are 
changing all of the time on all traits.
The Role Continuity  Principle
According to the role continuity principle, 
it is the consistent psychological mean -
ing of roles, not constituent, concrete situ -
ations in those roles, that cause continuity 
in personality traits over time. For example, 
people could move from one romantic re -