 11. Personality Development across the Life Course 267
changes in personality traits well past the 
termination of therapy and found that the 
changes experienced in therapy remained 
in place months and even years later. Most 
surprisingly, the majority of trait change 
happened relatively quickly—  in the first 
few weeks of therapy.
While it remains unclear the extent to 
which the changes imparted in therapy are 
the result of true trait change (Noordhof, 
Kamphuis, Sellbom, Eigenjuis, & Bagby, 
2018), these studies show that scores on 
standard measures of personality traits can 
and do change quickly—  much more quickly 
than any existing model or theory of per -
sonality would predict. While one may not 
be willing to ascribe transformative powers 
to psychotherapy, these studies demonstrate 
that it is possible to change personality traits 
and change them relatively fast, strongly supporting the idea that personality traits 
are plastic (see below).
Principles of  Personality  Development
With the ability to answer these pragmatic 
questions of personality development (How 
consistent are personality traits? Do per -
sonality traits change? If they change, why 
do personality traits change? And can we 
change personality traits?), we now have 
enough of an empirical foundation to syn -
thesize principles of personality develop -
ment that can be used to develop testable 
hypotheses and a conceptual, if not theoreti -
cal, foundation to the research on personal -
ity development. The following eight prin -
ciples were derived from a synthesis of the 
above findings (see Table 11.1).
TABLE 11.1.  Principles of Personality Development
Principle Definition Example researchStrength 
of evidence
Cumulative 
continuity principlePersonality traits increase in rank-order 
consistency until midlife.Ferguson (2010) Strong
Maturity principle People become more socially dominant, 
agreeable, conscientious, and emotionally stable 
with age.Donnellan, Conger, 
& Burzette (2007)Strong
Social investment 
principleInvesting in social institutions, such as age-
graded social roles, outside of the self is one 
of the driving mechanisms of personality 
development in general and greater maturity in 
particular.Bleidorn et al. 
(2013)Good
Corresponsive 
principleThe effect of life experience on personality 
development is to deepen the characteristics 
that lead people to those experiences in the first 
place.Roberts, Caspi, & 
Moffitt (2003)Moderate
Niche-picking 
principleThrough their personality traits, people create 
social environments and paths in their lives that 
help maintain their current trait levels.Roberts & Robins 
(2004)Weak
Plasticity principle Personality traits are open systems that can be 
influenced by the environment at any age.Lodi-Smith & 
Roberts (2012)Strong
Role continuity 
principleConsistent roles rather than consistent 
environments are the cause of continuity in 
personality over time.No example exists. Weak
Identity 
development 
principleWith age, the process of developing, committing 
to, and maintaining an identity leads to greater 
personality consistency.Lodi-Smith, 
Spain, Cologgi, & 
Roberts (2017)Weak