 8. The Neurobiology of Personality  195
does not translate directly into knowledge of 
causal mechanisms, because we are limited 
by the inability to manipulate personality 
traits experimentally. Even the most experi -
mental results of these methods, such as the 
effects of psychopharmacological manipula -
tion compared to placebo, can be linked to 
personality typically only through modera -
tion; that is, personality traits may be cor -
related with the magnitude of the effect of 
the manipulation. One must remember that 
such correlational findings can support hy -
potheses derived from mechanistic causal 
theories, but they do not provide direct evi -
dence of causal pathways. If one finds the 
size of some brain structure to be correlat -
ed with a trait, for example, one does not 
know whether variation in the size of that 
structure is a cause of variation in the trait 
or a consequence of variation in some other 
mechanisms that generate the trait.
Another important caveat about personal -
ity neuroscience research is that much of it 
has been statistically underpowered. Neuro -
imaging, especially, tends to be very expen -
sive, making it difficult to collect the large 
samples necessary for adequately powered 
research on individual differences. Nonethe -
less, underpowered research may be worse 
than no research at all, because low power 
increases the proportion of significant re -
sults that are false positives. Recent years 
have seen increasing sample sizes in person -
ality neuroscience, a good sign for the fu -
ture of the field. In our review, we attempt 
to focus on larger and more rigorous studies 
whenever possible, and to note when stud -
ies have problematically small samples. (For 
more in-depth discussions of methodologi -
cal issues in personality neuroscience, see 
Allen & De  Young, 2017; Yarkoni, 2015.)
Causal Implications 
of the Personality  Hierarchy
Personality traits are probabilistic descrip -
tions of relatively stable patterns of emotion, 
motivation, cognition, and behavior (De -
Young, 2015; Fleeson & Gallagher, 2009). 
Traits can be arranged hierarchically, based 
on their patterns of covariance, typically 
identified through factor analysis. Traits 
at higher levels of the hierarchy reflect the tendency of traits at lower levels to co-occur 
within individuals. The most well-  studied 
level of the hierarchy contains the so-  called 
“Big Five”—extraversion, neuroticism, 
openness/intellect, conscientiousness, and 
agreeableness—  which are the best validat -
ed broad dimensions of covariation among 
personality traits (John, Naumann, & Soto, 
2008; Markon, Krueger, & Watson, 2005). 
They also constitute the most widely used 
taxonomy for categorizing other traits, mak -
ing them useful for the purpose of reviewing 
a diverse research literature. Where possible, 
we describe studies in terms of the Big Five, 
even if they used questionnaires that did not 
employ standard Big Five labels.
At the bottom of the trait hierarchy are 
relatively narrow traits, often called facets.  
No consensus exists regarding the number 
and identity of facets subsumed beneath 
each of the Big Five. However, evidence from 
both genetic and standard factor analysis 
suggests another level of the hierarchy be -
tween the facets and the Big Five (Jang et al., 
2002; De  Young, Quilty, & Peterson, 2007), 
at which each of the Big Five can be divided 
into exactly two aspects,  representing the 
most important distinction for discriminant 
validity within each of the Big Five. In our 
review, we focus primarily on the Big Five, 
but we sometimes find it useful to consider 
the aspect-  level traits as well, when they 
show divergent associations with neural pa -
rameters.
Although the Big Five were originally con -
ceived as independent dimensions and the 
highest level of the trait hierarchy, they are 
in fact regularly intercorrelated, in a pattern 
that reveals the existence of two higher-  order 
factors or metatraits,  known as stability  (the 
shared variance of conscientiousness, agree -
ableness, and low neuroticism) and plastic -
ity (extraversion and openness/intellect). 
We have developed the hypothesis that the 
metatraits are influenced by global levels of 
serotonin and dopamine respectively (De -
Young, 2006, 2013), but we do not dwell on 
that hypothesis in this review, because most 
of the evidence comes from studies of the 
Big Five considered separately and so is de -
scribed within the sections below for each of 
the Big Five. (One important exception is a 
recent pharmacological manipulation study 
[N = 441], which directly confirmed that se -