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31
Cohort differences in personality
Gizem Hu
¨lu
¨r
University of Zurich, Zurich, Switzerland
The sociocultural environment surrounding us, including living circumstances, atti-
tudes, values, and material standards, is different today than it was years, decades,
and centuries ago and will probably be different in the future. According to lifespan
psychological and life course sociological perspectives (Baltes, Cornelius, &
Nesselroade, 1979; Bronfenbrenner, 1986; Caspi, 1987; Elder, 1974; Riley, 1973;
Ryder, 1965; Schaie, 1965), individual lives are embedded in and shaped by broader
societal and historical contexts. It is well documented that average levels of cognitive
performance considerably increased across the last century (Flynn, 2007; Pietschnig
&Voracek,2015
). Accumulating evidence suggests that other psychological traits,
such as Big Five personality (e.g., Mroczek & Spiro, 2003;Smits, Dolan, Vorst,
Wicherts, & Timmerman, 2011;Terracciano, McCrae, Brant, & Costa, 2005;
Twenge, 2000, 2001a), do also differ across generations. My objective in this chapter
is to highlight key issues in research on cohort differences in personality. Following
the broad definition of personality in this book, I will focus on a number of psycho-
logical individual difference characteristics, including Big Five traits, cognitive func-
tion, perceived control, self-esteem, and well-being; while acknowledging that it is
under debate whether some of these characteristics can be classified as personality
traits (for overviews, see DeYoung, 2011;Diener, 1996;Hooker & McAdams, 2003;
Kandler, Zimmermann & McAdams; 2014;Trzesniewski, Donnellan, & Roberts,
2003). This chapter is organized into four sections. First, I will give a definition of
cohort differences and highlight some methodological considerations. Second, I will
address why we expect personality to differ across cohorts. Third, I will give a selec-
tive overview of research on cohort differences in a number of psychological traits.
Fourth, I will outline open questions and avenues for future research.
What are cohort differences and how can we study them?
Conceptually, a cohort is a group of individuals who experience the same event at
the same time (Ryder, 1965). Studies of cohort differences in personality typically
examine differences between birth cohorts, that is, people born in the same historical
time who share common life experiences. Developmental scientists have long been
concerned with conceptual and methodological issues in the study of cohort differ-
ences (Baltes & Nesselroade, 1979; Schaie, 1965; Schaie & Baltes, 1975).
According to the age-cohort-period model (Schaie, 1965, 2011), each observation
Personality Development Across the Lifespan. DOI: http://dx.doi.org/10.1016/B978-0-12-804674-6.00031-4
©2017 Elsevier Inc. All rights reserved.
made at a specific time point is a function of age, cohort, and time period, or a com-
bination of these factors. Age effects refer to ontogenetic changes associated with
the chronological age of individuals. For example, conscientiousness typically
shows longitudinal increases in young adulthood (Roberts, Walton, & Viechtbauer,
2006), which can be considered an age effect that signifies maturation (Bleidorn,
2015). Cohort effects indicate how accumulated life experiences associated with
being born in a certain cohort shape personalities of individuals across the lifespan.
For example, if accumulated common life experiences of 20-year-olds born in the
1990s would make them less (or more) conscientious than 20-year-olds born in the
1980s, this would be considered a cohort effect. A period effect refers to effects
associated with living through a certain time period. Period effects may be of more
transient nature than cohort effects and may affect multiple cohorts. If unemploy-
ment (which is associated with declines in conscientiousness: Boyce, Wood, Daly,
& Sedikides, 2015), would increase during an economic crisis, lower levels of con-
scientiousness observed during this specific time period at population level would
be considered a period effect. Effects of age, cohort, and period are not independent
of one another; they can interact in multiple ways. For example, cohorts may differ
in how their conscientiousness changes with age: an age 3cohort interaction.
The majority of studies in this selective review utilized time lag analyses
(Schaie, 1965) to examine cohort differences. To examine cohort differences in
conscientiousness in a time-lag analysis, one would compare two sets of cross-
sectional data obtained from, for example, 20-year-olds in 2000 (born in 1980) and
20-year-olds in 2010 (born in 1990). In this type of design, age is being held con-
stant, while cohort and time period are confounded. Each birth cohort is examined
at a different time period. Although we sometimes interpret differences between
cross-sectional samples as differences between birth-cohorts (related to being born
in 1980 vs 1990), it is important to note that period effects (related to being in year
2000 vs 2010) may also be relevant. Cross-temporal meta-analysis (e.g., Twenge,
2000, 2001a) is a meta-analytic method based on the same principle as time lag
analyses, i.e., age is being held constant, while time of measurement varies. In
cross-temporal meta-analyses, researchers collect information on studies using the
same measure in similar age groups (e.g., college students) in different time peri-
ods. If scores differ over time, this would indicate existence of cohort (or period)
differences.
The longitudinal sequence method (also called cohort sequences; Schaie, 1965)
follows two (or more) cohorts over same age ranges, enabling to examine
agecohort interactions, e.g., whether changes in conscientiousness from age 20 to
30 are similar in those born in 1980 versus 1990. Again we sometimes interpret
these differences as age 3cohort interactions, but period effects could also be rele-
vant, as we observe each birth cohort growing older during a different time period
(e.g., 2000 to 2010 for the 1980 birth cohort, and 2010 to 2020 for the 1990 birth
cohort).
Studies utilizing time lag and longitudinal sequence designs offer insights into
generational shifts in personality, while specific events taking place at time of
measurement may also be relevant.
520 Personality Development Across the Lifespan
Why can we expect cohort differences in personality?
Industrialized countries have undergone many sociocultural changes over the last
century, some of which may be relevant for personality development across the life-
span. First, living circumstances changed considerably. Different cohorts went
through different experiences from early childhood to old age which may have
shaped their personalities in different ways. For example, after the post World-War II
Baby Boom, birth rates declined and families became smaller (e.g., Martin,
Hamilton, Osterman, Curtin, & Mathews, 2015). Furthermore, parenting styles may
have changed, with children being granted more autonomy for self-expression
(Rutherford, 2009). Both the quality of education and its quantity (i.e., years spent
in education) increased (Blair, Gamson, Thorne, & Baker, 2005; Schaie, Willis, &
Pennak, 2005). Work experiences also differed between cohorts. For example,
women’s labor force participation increased (Juhn & Potter, 2006). Also, relation-
ship experiences changed, with divorce becoming more common (see Amato, 2010)
and marriage and childbirth being postponed into later ages (Martin et al., 2015; US
Census Bureau, 2015). Cohorts probably also differed in experiences in old age:
People today live longer (Vaupel, 2010), and physical functioning has improved
(Crimmins, 2015). At the same time, diseases became more prevalent because
treatments increased length of life for those with disease (Crimmins, 2015). This
is of course only a selective list of changing life circumstances. All of these
factors mentioned above, including family structure (e.g., Zajonc, 1976), parenting
styles (e.g., Baumrind, 1971), education (e.g., Schaie et al., 2005), work experiences
(e.g., Schooler, Mulatu, Oates, 1999), intimate relationships (e.g., Hoppmann,
Gerstorf, Luszcz, 2011), length of life (e.g., Baltes & Smith, 2003), and health
(e.g., Wagner, Ram, Smith, & Gerstorf, 2015) were proposed to profoundly shape
individual development across the lifespan.
Second, people in different cohorts did not only make different experiences in
various life domains, they were also confronted with different social norms and
expectations. Thus the same personality traits may be associated with different con-
sequences in different cohorts. For example, George, Helson, and John (2011) found
that, among women born in the 1930s, being conscientious was unrelated to work
involvement, satisfaction with work, or status level in young and middle adulthood.
Instead, higher conscientiousness predicted commitment to wife and mother roles.
The authors argued that due to shifts in social norms, they would expect conscien-
tious women today to pursue both career and family goals. This would mean that
being conscientious would relate to different outcomes based on societal expecta-
tions. Also, perceptions of what is considered a desirable personality trait may
change across cohorts: For example, Liu, Chen, Li, and French (2012) argued that
while shyness may have been considered a positive personality trait in traditional
Chinese society valuing group harmony, it may become a less desirable trait in a
market economy. In line with this reasoning, they found that shyness was associated
with positive developmental outcomes such as leadership and academic achievement
in 1994. However, in 2008, shyness predicted only negative outcomes, such as lower
521Cohort differences in personality
peer preference or higher loneliness. Taken together, these findings suggest that the
same personality traits may impact individuals’ lives differently depending on the
historical time they live in.
In conclusion, several factors proposed to shape personality development
changed over recent decades and personality traits probably also changed in how
adaptive they are for certain outcomes. As a result, a growing body of research
examines cohort differences in personality.
What is known about cohort differences in personality?
In this section, I will give a selective overview of research on cohort differences in
personality. As noted in the introduction, following the broad definition of personal-
ity in this book (see Part IV), I refer to personality as a wide range of psychological
traits that show meaningful between-person differences.
Big Five personality. The Five-Factor Theory of personality (McCrae & Costa,
2003) is currently one of the most influential theories in personality psychology
(see Mo
˜ttus, Chapter 7). Although research on Big Five personality pays considerable
attention to the broader sociocultural context in terms of cross-cultural differences
(e.g., Costa, Terracciano, & McCrae, 2001), relatively few studies examined how per-
sonality differs across historical contexts.
Twenge (2000, 2001a) was among the first to examine cohort differences in Big
Five traits and demonstrated increases in extraversion and neuroticism in US college
students based on cross-temporal meta-analyses. Twenge’s (2001a) finding of
increased extraversion was replicated in other studies. Extraversion (or some of its
facets) increased across cohorts in predominantly middle-aged and older US
(Mroczek & Spiro, 2003; Terracciano et al., 2005)andSwedish(Billstedt et al., 2013)
samples and in Dutch freshman psychology students (Smits et al., 2011). Twenge
(2001a) provided several explanations for increasing extraversion, including emphasis
on social skills in schools and the shift to a service economy. These experiences may
also explain higher extraversion among later-born cohorts of middle-aged and older
individuals, who may be able to maintain previous levels of extraversion because of
historical improvements in health (Crimmins, 2015; Wagner et al., 2015).
Research is less conclusive with regard to cohort differences in neuroticism in
young adults. Twenge (2000) reports increases in neuroticism among US college
students, whereas Smits et al. (2011) found declines among Dutch students. In
middle-aged and older adults, available findings point to a historical decline in
neuroticism (Mroczek & Spiro, 2003; Terracciano et al., 2005; but see Billstedt
et al., 2013; no significant difference). These findings are consistent with studies
observing higher levels of psychosocial function among later-born cohorts of older
adults (e.g., Gerstorf et al., 2015;Hu
¨lu
¨r et al., 2016;Sutin et al., 2013).
Few studies examined cohort differences in other Big Five traits (for exceptions,
see Smits et al., 2011;Terracciano et al., 2005). Furthermore, relatively less is known
about cohort differences in age-related change. Terracciano et al. (2005) found no
522 Personality Development Across the Lifespan
cohort differences in change, whereas Mroczek and Spiro (2003) found that later-
born participants showed relatively less decline in extraversion and more decline in
neuroticism. It is important to note that the different birth cohorts in both studies
showed little overlap in the age range examined. For example, in Mroczek and
Spiro’s study (2003), because of the study design, differences between an earlier-
born (18971919) and a later-born (192029) cohort in age-related trajectories
could only be observed between 70 and 75 years of age. It is an open question
whether similar cohort differences in longitudinal change can be found at other ages.
Cognitive performance. It is well documented that cognitive test scores, espe-
cially in tests of fluid cognitive performance, increased dramatically in many industri-
alized countries across the last century (Flynn, 2007:Pietschnig & Voracek, 2015).
Several factors have been proposed to underlie this development, including improve-
ments in nutrition and healthcare (Lynn, 1998), education (Schaie et al., 2005), or
resources available per child (Sundet, Borren, & Tambs, 2008), all of which are
related to economic prosperity. In line with this reasoning, increases in test scores
were associated with gross domestic product growth per capita (Pietschnig &
Voracek, 2015). Furthermore, test scores increased more steeply in countries that
started industrializing more recently (Nisbett et al., 2012; Wongupparaj, Kumari, &
Morris, 2015), suggesting that cohort differences in cognitive performance could be
based on broader societal changes resulting from industrialization.
The research outlined above mainly focused on historical gains in children, ado-
lescents, and young adults. Because of demographic changes toward an older popu-
lation, it is important to know whether these gains are maintained in older ages.
Accumulating evidence suggests that historical gains are also observed among older
adults (e.g., Bowles, Grimm, & McArdle, 2005;Christensen et al., 2013;Gerstorf
et al., 2015;Gerstorf, Ram, Hoppmann, Willis, & Schaie, 2011;Skirbekk,
Stonawski, Bonsang, & Staudinger, 2013). However, it is less clear whether cohort
differences also exist in rates of change, that is, whether later cohorts show less
steep decline of cognitive performance. So far, available findings are inconclusive
and show that older adults in later-born cohorts show less (Dodge, Zhu, Lee, Chang,
& Ganguli, 2014; Gerstorf et al., 2011; Schaie et al., 2005), equal (Finkel, Reynolds,
McArdle, & Pedersen, 2007; Zelinski & Kennison, 2007), or more decline
(Karlsson, Thorvaldsson, Skoog, Gudmundsson, & Johansson, 2015). Furthermore,
initial evidence suggests that advantages of later cohorts may disappear at the end of
life (Gerstorf et al., 2011; Hu
¨lu
¨r, Infurna, Ram, & Gerstorf, 2013).
Perceived control. Perceived control, i.e., the general sense of having control
over one’s life (Rotter, 1966; Skinner, 1995) can apply to multiple situations and
may be considered a personality characteristic (see Specht, Egloff, & Schmuckle,
2013). Control beliefs can be differentiated into two independent dimensions
(Lachman, Neupert, & Agrigoroaei, 2011). People with higher internal control
beliefs think that their life outcomes depend on their own efforts, while those with
higher external control beliefs think that external circumstances are crucial. An
individual may hold the belief that working hard will lead to success, while also
thinking that external factors beyond their control may hamper reaching important
goals. Thus a person may be high both in external and internal control beliefs.
523Cohort differences in personality
Research on cohort differences in high school and college students is incon-
clusive. In a cross-temporal meta-analysis, Twenge, Zhang, and Im (2004) found
that control beliefs became more external between 1960 and 2002 in college
students. However, Trzesniewski and Donnellan (2010) showed that perceived
control remained stable between 1976 and 2006 among high school seniors.
Methodological differences may have contributed to the discrepancy: While
Trzesniewski and Donnellan’s (2010) measure assessed perceived personal con-
trol (“When I make plans, I am almost certain I can make them work”), Twenge
et al. (2004) measure included items assessing perceived control both over
personal and societal outcomes (“In the long run the people are responsible for
bad government on a national as well as on a local level”). It is possible that
perceived control over societal outcomes is more susceptible to historical change.
It is also important to note that both studies used one-dimensional scales of
perceived control. Individuals could be high in either external or internal control
beliefs, but not in both.
In a recent study, we found that older adults today reported substantially lower
levels of external control compared to older adults 20 years ago, while no cohort
difference was found in internal control beliefs (Hu
¨lu
¨r et al., 2016). One potential
explanation is that lives of earlier-born participants were impacted more by major
historical events on which most of them probably had no or little direct personal
control, such as World War II. Thus it is an open question whether our finding is
specific to a geographical location (Berlin or Germany) or whether similar trends
exist elsewhere. Furthermore, declines in religiosity across cohorts may be relevant
because more religious individuals may believe that events are predetermined
(Fiori, Brown, Cortina, & Antonucci, 2006; Wolf, 2008). Although we controlled
for religious affiliation, cohort differences in subjective importance of religion may
still have existed.
Subjective well-being. Subjective well-being can be considered as a summary
measure of how well an individual is doing in multiple life domains (Diener, 1984).
Well-being involves trait-like components and shows some degree of stability
across the lifespan (Diener & Lucas, 1999). Studies examining population trends in
large-scale annual cross-sectional surveys such as the General Social Survey found
that cohort and period effects were relatively minor (e.g., Blanchflower & Oswald,
2004;Twenge, Sherman, & Lyubomirsky, 2016;Yang, 2008). Furthermore,
historical trends differed by age group with adolescents showing slight increases
and people older than 30 years showing slight declines between 1972 and 2014
(Twenge et al., 2016). In population-based samples of high school seniors between
1976 and 2006, happiness and life satisfaction were relatively stable (Trzesniewski
& Donnellan, 2010). Also, differences in well-being based on gender and ethnicity
declined across time (Yang, 2008).
Studies of mental health trends in high school and college students suggest that
more subtle depressive symptoms increased, whereas more severe symptoms such
as suicidal ideation (or suicide rates) declined (Twenge, 2015; Twenge et al., 2010).
Also, access to mental healthcare increased, which may explain declines in more
severe mental health issues (Twenge, 2015). At the population level, little evidence
524 Personality Development Across the Lifespan
for cohort differences in mental health was found in the annual cross-sectional
National Psychiatric Morbidity Surveys in England between 1993 and 2007
(Spiers et al., 2011, 2012). Taken together, available evidence points to relative
stability of well-being across cohorts.
Studies focusing on older adults generally find that later-born cohorts report
higher levels of well-being, conceptualized as higher life satisfaction or fewer
depressive symptoms (Gerstorf et al., 2015; Hu
¨lu
¨r, Ram, & Gerstorf, 2015; Sutin
et al., 2013; Zivin, Pirraglia, McCammon, Langa, & Vijan, 2013; but see Schilling,
2005). However, initial evidence suggests that cohort-related increases in well-
being may disappear at the end of life (Hu
¨lu
¨r et al., 2015).
It is important to note that these findings were obtained in participants from the
United States, the United Kingdom, and Germany. Different trends may exist in
other sociocultural contexts. For example, studies from China show that depressive
symptoms increased both in adolescents and older adults over the last two decades
(Shao et al., 2013; Xin, Niu, & Chi, 2012). Authors of both studies argue that chal-
lenges to traditional family structures due to industrialization and urbanization may
be relevant. Thus different historical trends may be at play based on geographical
and sociocultural context.
Self-esteem. Self-esteem refers to an individual’s global evaluations and feelings
of his or her own worth (Orth & Robins, 2014). Cross-temporal meta-analyses
showed increases in elementary, middle school, and college students’ self-esteem as
measured by the Rosenberg Self-Esteem Scale (Rosenberg, 1965) over the recent
decades (Gentile, Twenge, & Campbell, 2010; Twenge & Campbell, 2001).
Findings were inconclusive with regard to high school students’ self-esteem: While
Gentile et al. (2010) found increases, Twenge and Campbell (2001) observed an
increase only in boys. Furthermore, in a time-lag analysis with population-based
samples of high school seniors between 1976 and 2006, Trzesniewski and
Donnellan (2010) found no cohort differences in self-esteem.
Available evidence on age-related trajectories of self-esteem suggests that
cohorts did not differ in trajectories of change (Erol & Orth, 2011; Orth,
Trzesniewski, & Robins, 2010; Orth, Robins & Widaman, 2012). Because of longi-
tudinal design features, cohorts did not completely overlap in age ranges examined.
For example, Orth et al. (2012) examined differences in self-esteem trajectories
between four generations (children, parents, grandparents, and great-grandparents).
With each participant contributing data over up to 12 years, some generations
showed overlap in longitudinal age ranges examined, whereas others did not.
Children showed some overlap with parents (children: 16 to 47 years, parents: 19 to
68 years), whereas they showed little overlap with grandparents (41 to 90 years)
and no overlap with great grandparents (61 to 97 years). Thus the study provides
insights on cohort differences within overlapping ages; however, it is possible that
differences may have existed outside these age ranges.
In conclusion, available evidence indicates that self-esteem was either stable
or increased over the last two to three decades, while age-related trajectories were
similar across cohorts. Again, these studies were based on US samples and different
trends were observed elsewhere (e.g., China: Liu & Xin, 2015). Also, given
525Cohort differences in personality
that some aspects of psychosocial function improved across cohorts in old age
(e.g., Gerstorf et al., 2015;Hu
¨lu
¨r et al., 2016;Sutin et al., 2013), it is an open ques-
tion whether similar trends will be observed for self-esteem.
Open questions
Taken together, available evidence clearly supports the idea that psychological traits
are shaped by the time periods people live in. As psychologists were usually inter-
ested in identifying processes that generalize across historical periods, cohort differ-
ences were often considered nuisances (Caspi, 1987). Thus with the exception of
cohort differences in cognition and well-being, empirical evidence is beginning to
accumulate and a number of questions are still open. According to lifespan psycho-
logical and life course sociological developmental perspectives, individual develop-
ment is shaped by both historic and ontogenetic processes and their interactions. As
Caspi (1987) noted, “the changing environmental context of development must be
thoroughly understood and analyzed before an adequate account of individual
behavior is possible” (p. 1211) (see also Caspi & Roberts, 2001). Below, I will
address some open questions and outline avenues for future research.
Linking cohort differences to developmental theory. Since various societal
and cultural changes took place over recent decades, it is not too difficult to find
post hoc explanations for any finding. For example, because more employees
engage in teamwork (Inanc, Felstead, Gallie, & Green, 2013), one may predict
increases in agreeableness, as it may be beneficial for working in groups (Barrick,
Stewart, Neubert, & Mount, 1998). On the other hand, declines in collectivistic
values (Park, Twenge, & Greenfield, 2014) may predict declines in agreeableness,
as agreeableness is related to collectivism (Benet-Martinez & Karakitapoglu-
Aygu
¨n, 2003). Thus it is becoming important to tie research on cohort differences
more closely to theory. For example, the theory of emerging adulthood proposes
that younger adults are beginning to enter adult roles at later ages (Arnett, 2000).
Based on this theory, one could develop the hypothesis that cohorts of late adoles-
cents and young adults differ in developmental trajectories of personality traits that
are considered to signify maturation (Bleidorn, 2015), such as emotional stability,
conscientiousness, and agreeableness. Fig. 31.1 illustrates two hypotheses based on
the theory of emerging adulthood (Arnett, 2000). In Panel A, it is hypothesized that
personality maturation (illustrated here as increase in conscientiousness) is post-
poned to a later age for individuals in the later-born cohort, because they enter adult
roles at a later age. Once processes of personality maturation set in, both cohorts
follow the same trajectory with a similar rate of increase. In contrast, in Panel B,
both cohorts start personality maturation at the same age. However, the later-born
cohort shows slower increases in conscientiousness than the earlier-born cohort,
because of, for example, a prolonged period of identity exploration.
Also, hypotheses of compression of morbidity versus manufactured survival make
different predictions regarding cohort differences in old age (Fries, 1980, 2005;
Olshansky, Hayflick, & Carnes, 2002; for an overview and illustration, see Hu
¨lu
¨r,
526 Personality Development Across the Lifespan
Ram, & Gerstorf, 2016). For example, according to the compression of morbidity
hypothesis, older adults today would live longer lives and experience declines in
function at relatively later ages compared to previous cohorts. On the other hand,
according to the manufactured survival hypothesis, increased length of life would go
Figure 31.1 A graphical illustration of two hypotheses based on the theory of emerging
adulthood (Arnett, 2000). Panel A: According to the hypothesis illustrated in Panel A,
personality maturation (here: increase in conscientiousness) shifted to later ages in the later-
born cohort (in black) as compared to the earlier-born cohort (in gray). However, once
processes of personality maturation set in, both cohorts undergo this process at the same rate.
Panel B: According to the hypothesis illustrated in Panel B, personality maturation starts at
the same age for both cohorts. However, personality maturation occurs at a faster rate for the
earlier-born cohort (in gray) as compared to the later-born cohort (in black) that experiences
a prolonged period of identity formation. Please note that for the sake of simplicity, this
illustration assumes that levels of conscientiousness at baseline (e.g., childhood or
adolescence conscientiousness) and levels of conscientiousness after maturation are equal in
both cohorts. Both of these assumptions are testable in empirical data.
527Cohort differences in personality
along with expansion of morbidity, that is, older adults today would spend more
years in poor health. As of yet, available evidence does not conclusively support one
or the other scenario (see Vaupel, 2010). Potentially, these historical trends carry
important implications for personality development, since health has been proposed
to affect personality late in life (Wagner et al., 2015). Taken together, a number of
specific hypotheses on cohort differences can be derived based on developmental the-
ory to further our understanding of personality development across the lifespan.
Subgroup differences. Some historical developments differentially impacted spe-
cific subgroups of the population. For example, Twenge (2001b) found that women’s
assertiveness varied across historical time synchronously with women’s social status
in society (see also Andre
´et al., 2010), while similar trends were not observed in
men. With regard to cognitive performance, researchers extensively studied whether
historical improvements were stronger at the lower or upper end of the test score dis-
tribution (Nisbett et al., 2012). Subgroup differences in test score gains can provide
insights into potential mechanisms responsible for historical developments. For exam-
ple, Nisbett et al. (2012) argued that if cohort differences were based on improve-
ments in nutrition, this should translate into stronger gains at the lower end of the
distribution, because upper classes probably had access to a nutritious diet as well in
the past. In summary, the study of subgroup differences can provide important
insights into mechanisms involved in individual development.
Issues of measurement. Another open question is whether observed historical
differences reflect changes in the underlying psychological traits or if they are
related to response behaviors. This issue has been thoroughly discussed in the study
of cohort differences in cognitive performance: Do the observed increases in tests
scores reflect cohort differences at the level of cognitive performance in single
tests, or at the level of the underlying ability that the tests were designed to measure
(Pietschnig & Voracek, 2015; Wicherts et al., 2004)? Available evidence suggests
that test-taking skills may partially explain historical trends; however, substantial
improvements nevertheless remain after controlling for cohort differences in test-
taking behavior (see Pietschnig & Voracek, 2015).
Also, cohorts may differ in how they perceive and evaluate their personalities.
For example, Costa et al. (2001) surprisingly found larger gender differences in
personality in societies with higher gender equality. Research suggests that peo-
ple more often engage in between-gender comparisons in more gender-equal
societies, whereas in societies with more rigid gender roles same-gender compar-
isons are more common (Guimond et al., 2007). With gender roles becoming
more egalitarian over time, between-gender comparisons may be more frequent
among later-born cohorts. That women’s self-reports of assertiveness neverthe-
less increased from the late 1960s to 1990s (Twenge, 2001b)mayimplythat
increases in the underlying personality trait of assertiveness may be even more
substantial.
Tests of measurement invariance, i.e., whether instruments are suited to measure
the same underlying construct in different cohorts, may help with some of these
issues (see Smits et al., 2011). For example, Smits et al. (2011) found that the item
“I think it is important to dress to fit the occasion” was biased with respect to cohort.
528 Personality Development Across the Lifespan
This finding is probably based on changing social norms regarding dress codes
(see Smits et al., 2011). The decision to exclude such biased items had impact on the
size of observed cohort differences (Smits et al., 2011). In summary, it is a largely
open question whether observed historical changes took place at the level of under-
lying traits. Studies of measurement invariance across cohorts can provide some
information in this regard.
Conclusion
The sociocultural environment surrounding us has undergone many important his-
torical changes in recent decades. As outlined in this chapter, cohort differences
were documented in a number of between-person difference characteristics. From a
theoretical perspective, these findings clearly demonstrate that personality is shaped
by the broader historical and sociocultural context individuals are embedded in.
Thus to better understand personality development across the lifespan, future
research needs to consider both historical and ontogenetic processes and their
interplay.
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