196 II. BIOLOGICAL  FOuNDATIONS
rotonin function is associated with stability; 
Wright et al. [2019].)
We have now mentioned four levels of the 
trait hierarchy. In descending order, they are 
metatraits, the Big Five, aspects, and facets. 
What are the causal implications of this hi -
erarchical structure? Perhaps most impor -
tantly, factor analysis cannot indicate the 
nature or number of the forces that cause 
traits to covary. The existence of a single 
statistical factor does not indicate the exis -
tence of a single cause. It merely indicates 
that some force or set of forces, of any num -
ber, causes the lower-level traits within that 
factor to covary. In other words, factors are 
merely descriptive entities; they are not ex -
planatory. The most plausible models link -
ing personality to brain function indicate 
that the variance of any one trait dimension 
is likely to be caused by multiple neural pa -
rameters (Allen & De  Young, 2017; Yarkoni, 
2015; Zuckerman, 2005). Nonetheless, the 
hierarchy does suggest one useful principle: 
that at any given level, some forces affect 
each trait specifically, but other forces affect 
all traits that are grouped together at the 
next higher level. For example, the forces in -
fluencing extraversion cause its two aspects, 
assertiveness and enthusiasm, to covary, but 
other forces exist that differentiate the two 
aspects. Each level of the hierarchy is likely 
to have its own set of neural correlates.
Another important caveat is that the trait 
hierarchy, as typically represented, is an 
oversimplification, because traits do not 
have simple structure, especially below the 
level of the Big Five. In other words, aspects 
and facets covary with each other in ways not 
indicated by the standard hierarchical dia -
grams. For example, although extraversion 
and agreeableness are unrelated, lower-level 
traits within extraversion show a complex 
pattern of positive and negative associations 
with lower-level traits within agreeableness 
(De Young, Weisberg, Quilty, & Peterson, 
2013). Thus, in addition to the fact that a 
single trait is likely to have multiple causes, 
even more causal connections exist than are 
suggested by our standard hierarchical mod -
els of traits. The neurobiology of personality 
is extremely complex, and we are just begin -
ning to scratch the surface of it.
Nonetheless, we believe that progress can 
be made, especially by drawing on theories of the psychological functions that unify 
personality traits (e.g., Dennisen & Penke, 
2008; De  Young, 2015; Nettle, 2006). If a 
psychological function can be identified un -
derlying a trait, then we can rely on exist -
ing knowledge of brain function to identify 
candidate brain systems that contribute to 
that psychological function and, hence, are 
likely to be involved in the trait in question. 
Early attempts at personality neuroscience 
tended to focus on particular brain regions 
or single neurotransmitters. Neuroscience 
research, in general, has been moving away 
from a focus on individual regions of the 
brain to a focus on networks of interacting 
regions that cooperate to carry out various 
functions. Due to limitations of space, we 
restrict our review of existing knowledge in 
personality neuroscience to an overview of 
the most compelling findings, which typi -
cally are not only those that are especially 
methodologically rigorous but also those 
that link traits to theoretically relevant brain 
systems (for a more comprehensive review, 
see Allen & De  Young, 2017). Table 8.1 pro -
vides an overview of most of the neural cor -
relates discussed in the chapter.
Extraversion
Current theories of the neural basis of ex -
traversion stem largely from the work of 
Depue and Collins (1999), who linked it to 
dopamine and the brain’s reward system. 
They were inspired by the work of Gray 
(1973; Pickering & Gray, 1999), a student 
of Eysenck, who developed an influential 
approach to personality neuroscience called 
reinforcement sensitivity theory, which at -
tempted to connect personality to the neural 
processes that govern reward and punish -
ment. Gray referred to the reward system as 
the behavioral approach system (BAS). Al -
though Gray originally speculated that the 
trait most closely reflecting BAS sensitivity 
might be impulsivity, subsequent research 
indicates that extraversion is a better candi -
date, and that questionnaires explicitly de -
signed to assess BAS sensitivity are typically 
most closely aligned with extraversion (Pick -
ering, 2004; Quilty, De  Young, Oakman, & 
Bagby, 2014; Wacker, Mueller, Hennig, & 
Stemmler, 2012; Zelenski & Larsen, 1999). 