Chapter 13 McCrae and Costa’s Five-Factor Trait Theory 397Second, the origin postulate takes a clear if somewhat controversial stance:
All personality traits are the result solely of endogenous (internal) forces, such as
genetics, hormones, and brain structures. In other words, the family environment
plays no role in creating basic tendencies (but again, recall that personality traits
are not synonymous with personality as a whole). Figure 13.3 shows but one
causal arrow going from biological bases to basic tendencies. Such a claim is
based mostly on the robust findings from behavioral genetics that the five dimen-
sions of personality can be almost exclusively explained (about 50% each) by two
factors, namely genetics and nonshared environment (Hamer & Copeland, 1998;
Loehlin, 1992; Plomin & Caspi, 1999). Genetic influence is demonstrated by what
behavioral geneticists refer to as heritability coefficients and comes out of the
research on adoption studies and twin studies. Heritability addresses the question
of what is the difference in the correlation on a given personality trait between
individuals who are genetically identical (identical twins) and those who share
only about 50% of their genes (all other siblings). If genes played no role in shap-
ing traits, no differences would be found in correlations between people who vary
in their degree of genetic similarity. Identical and fraternal twins would be just
as similar or just as different. Evidence indicates that identical twins, even if
reared in different environments, show greater similarity in personality than other
siblings. And in the case of most personality traits, the degree of similarity sug-
gests that about 50% of the variability in personality is due to heritability or
genetics. Most of the remaining 50% is explained by nonshared experiences of
siblings of varying ages; that is, siblings usually have different experiences,
friends, and teachers. For instance, parents change their own parenting behaviors
with time and experience. Thus, a child born three or four years after another is
being raised in a somewhat different environment. More recent research has
uncovered genetic regions associated with all five dimensions of personality (de
Moor et al., 2012).
Third, the development postulate assumes that traits develop and change
through childhood, but in adolescence their development slows, and by early to
mid-adulthood (roughly age 30), change in personality nearly stops altogether
(Costa & McCrae, 1994; Costa, McCrae, & Arenberg, 1980).
McCrae and Costa (2003) speculated that there may be some evolutionary
and adaptive reasons for these changes: When people are young and establishing
their relationships and careers, high E, O, and even N would be beneficial. As
people mature and become settled, these traits are no longer as adaptive as they
were earlier. Moreover, increases in agreeableness and conscientiousness might be
helpful as people age. In our section on research, we discuss stability of traits dur-
ing adulthood.
Finally, the structure postulate states that traits are organized hierarchically
from narrow and specific to broad and general, just as Eysenck (1990) had sug-
gested. This postulate grows out of McCrae and Costa’s long-held position that
the number of personality dimensions is five and only five. This number is more
than the three hypothesized by Eysenck and considerably fewer than 35 found by
Cattell. With the structure postulate, McCrae and Costa and other five-factor
theorists converge on five as the answer to the long-standing debate among factor
theorists.