160 II. BIOLOGICAL  FOuNDATIONS
been identified, effect sizes typically are 
very low (e.g., < .05), and failure to replicate 
higher values found in particular studies 
is more common than not (Sanchez-  Roige, 
Gray, MacKillop, Chen, & Palmer, 2018). 
Some researchers have examined personal -
ity scales’ facets and found that gene-trait 
relations differ across facets (e.g., Sanchez- 
Roige et al., 2018), which raises the question 
of whether there is an optimal level of the 
personality trait hierarchy for studying these 
associations. One possibility is that increas -
ing levels of specificity will continue to find 
differential gene–trait relations, resulting in 
considerable complexity. However, at some 
point, the decreased reliability of more and 
more specific trait nuances will introduce 
new complications into research on gene–
trait relations.
The Temporal Stability of  Personality
The Genetic and Environmental Basis of Stability
A discussion of the stability of personality 
over time may seem superfluous in light of 
our earlier review of heritability data. In -
deed, a popular misconception is that be -
cause we are born with a full complement 
of genes, their influence necessarily must be 
stable and invariant throughout the lifespan 
which, in turn, implies that there should be 
a direct, positive correlation between the 
heritability of a trait and its temporal sta -
bility (for discussions, see Pedersen, 1993; 
Viken et al., 1994). In actuality, however, 
genes can be a source of both stability and 
change in personality (Kandler, 2012). In -
deed, age-  specific genes (e.g., a gene that 
influences temperament during adolescence 
but is quiescent during adulthood) gener -
ally can be expected to lead to instability in 
individual differences over time (McGue et 
al., 1993; Pedersen, 1993). Conversely, un -
changing aspects of the environment (e.g., a 
long-term career or marriage) may well play 
an important role in maintaining the stabil -
ity of temperament. Consequently, there is 
no necessary correlation between stability 
and heritability.
That said, we also must acknowledge that 
available data suggest that the popular view 
is reasonably accurate after all: Genes do 
appear to be the major source of observed stability in temperament, whereas environ -
mental factors are primarily responsible 
for change (Kandler, 2012; McGue et al., 
1993; Pedersen, 1993; Viken et al., 1994). 
For instance, McGue et al. (1993) estimated 
the heritability of the stable component to 
be .71, .89, and .89 for N/NE, E/PE, and 
DvC, respectively. Note that although these 
values demonstrate that genetic factors are 
overwhelmingly responsible for phenotypic 
stability, they also indicate that the un -
shared environment has a nontrivial influ -
ence on the observed continuity of tempera -
ment. Conversely, McGue et al. found that 
although the unshared environment was 
primarily responsible for observed changes 
on these traits, genetic factors also played 
a moderate role in the instability of N/NE 
and DvC, and a more modest role in pro -
ducing changes on E/PE. They suggested 
these genetic sources of instability may be 
linked to the observed declines in heritabil -
ity that were discussed earlier. For example, 
age- specific genes may exert a significant 
influence on N/NE in adolescence but then 
decline in importance during adulthood, 
leading to both lower heritabilities and phe -
notypic instability in N/NE over time.
These data establish that genes are pri -
marily responsible for stability, but they do 
not address the issue of stability itself. How 
stable are the major dimensions of tempera -
ment over time? In discussing this issue, it 
is useful to distinguish three different types 
of stability: mean-level stability, rank-order 
stability,  and structural stability  (see Peder -
sen, 1993).
Mean‑Level  Stability
Mean-level stability concerns whether av -
erage levels of a trait change systematically 
with age. For example, do people generally 
become more cautious and constrained as 
they grow older? Reviews of these data have 
yielded several clear conclusions (Caspi, 
Roberts, & Shiner, 2005; Roberts, Walton, 
& Viechtbauer, 2006; Vaidya, Gray, Haig, 
& Watson, 2002; Watson & Humrichouse, 
2006).
First, trait levels of N/NE show a signifi -
cant decline with age, the bulk of which oc -
curs in adolescence and early adulthood; 
nevertheless, decreases continue to be seen 