268 III. DEvELOPMENT
The Cumulative Continuity  Principle
According to the cumulative continuity 
principle, personality traits increase in rank-
order consistency throughout the lifespan 
(Roberts & Caspi, 2003). Rank-order con -
sistency reaches its highest point between 
ages 50 and 60, after which there is a plateau 
or decrease (Roberts & DelVecchio, 2000). 
The evidence for the cumulative continuity 
principle is stronger than the evidence for 
most of the remaining principles of the neo- 
socioanalytic model. We derived this prin -
ciple from a meta-  analysis of the rank-order 
consistency of personality traits (Roberts 
& DelVecchio, 2000), and a second meta- 
analysis confirmed the findings, using a 
new set of longitudinal studies (Ferguson, 
2010). Recent empirical research even finds 
evidence for the cumulative continuity prin -
ciple in adolescence (Borghuis et al., 2017). 
Despite recent work (Costello, Srivastava, & 
Saucier, 2017) that failed to find evidence 
for a statistically significant increase in sta -
bility (the increases tend to be small decade 
to decade), we believe that this principle is 
akin to the “first law” of behavior genetics 
(i.e., virtually all phenotypes are heritable; 
Turkheimer, 2000). Based on the evidence, 
the first law of personality development 
should be that, at a population level, rank-
order consistency increases as people grow 
older until midlife (even in self-  esteem; Trz -
esniewski, Donnellan, & Robins, 2003).
The Maturity  Principle
According to the maturity principle, people 
become more psychologically mature as they 
get older—  maturity is defined as increasing 
agreeableness, conscientiousness, and emo -
tionally stability (Hogan & Roberts, 2004). 
Findings in both cross-  sectional and longitu -
dinal data support this principle: As people 
age, they tend to increase in these three per -
sonality traits (Donnellan & Lucas, 2008; 
Roberts et al., 2006; Srivastava et al., 2003).
The recent longitudinal evidence in sup -
port of the maturity principle encompasses 
data from many different research teams 
and multiple longitudinal studies from a va -
riety of countries. As previously mentioned, 
evidence for this principle comes from lon -
gitudinal studies from Iowa (Donnellan et al., 2007), Minnesota (Durbin et al., 2016), 
Germany (Lüdtke et al., 2011), Finland (Jo -
seffsson et al., 2013), and Italy (Vecchione 
et al., 2012). Like the cumulative continuity 
principle, the maturity principle has strong 
support and could be considered the second 
law of personality trait development.
The Social Investment  Principle
The social investment principle helps explain 
the reasons for greater maturity with age. 
According to this principle, young adult -
hood is a prime time for personality trait 
change, because it is the age when many in -
dividuals newly invest in conventional social 
roles (e.g., spouse, employee) that have at -
tached to them experiences and expectations 
for being agreeable, conscientious, and emo -
tionally stable (Roberts, Wood, & Smith, 
2005). Put more simply, young adults’ per -
sonalities change due to their new commit -
ment to adult social roles (Lodi-Smith & 
Roberts, 2007). The social investment prin -
ciple describes a normative process (Helson, 
Kwan, John, & Jones, 2002); in societies all 
over the world, people commit themselves in 
young adulthood to adult roles associated 
with family, work, and community.
Findings from longitudinal data have 
shown that changes in investment in adult 
social roles are correlated with changes in 
personality traits. In a study of age differ -
ences in personality trait development in 
62 countries, the participants who invested 
in adult social roles at a younger age devel -
oped more quickly in terms of personality in 
a manner consistent with the social invest -
ment principle (Bleidorn et al., 2013). In two 
longitudinal studies, people who increased 
their commitment to work also increased in 
conscientiousness (Hudson, Roberts, & Lo -
di-Smith, 2012; Hudson & Roberts, 2014). 
Similarly, the results of a 3-year longitudinal 
study of Finnish students indicated that the 
first-time initiation of a career or job was 
linked to an increase in conscientiousness 
(Leikas & Salmela-  Aro, 2015). Analogous 
associations have been found for relation -
ship experiences. For example, remaining 
in a stable relationship is associated with 
increases in conscientiousness (Jonkmann, 
Thoemmes, Lüdtke, & Trautwein, 2014; 
Robins et al., 2002), and becoming increas -