 8. The Neurobiology of Personality  207
was negatively associated with serotonin 
function in men but not in women (Manuck 
et al., 1998), whereas a 2-month trial on 
an SSRI significantly reduced aggression 
in women but not in men (Kamarck et al., 
2009).
Future  Directions
In this chapter, we have presented a rela -
tively brief overview of the state of person -
ality neuroscience, using the Big Five as an 
organizing framework. Brevity may have 
contributed to giving the impression of more 
coherence than actually exists in what is a 
young discipline with a great deal of incon -
sistency and uncertainty in its results. None 
of the findings reviewed here should be con -
sidered to have the status of fact; everything 
needs to be replicated and further investigat -
ed. Still, we believe this is an exciting time 
for personality neuroscience, when research 
methods are improving and knowledge is in -
creasing rapidly. We close with several sug -
gestions for those who wish to contribute to 
the field.
First and foremost, samples should be rea -
sonably large. Many of the effects of interest 
are likely to be equivalent to correlations of 
about .25 or less, and such correlations can -
not be stably estimated without large sam -
ples (Schönbrodt & Perugini, 2013). Note 
that effects of this size are not negligible. 
They are comparable to the effects typically 
found in personality and the rest of psychol -
ogy (Gignac & Szodorai, 2016; Hemphill, 
2003), and these are typical for situations 
in which there are many causal influences 
on the phenomena under study. With more 
than a small number of causal determinants 
of some measure, the correlation between 
any one of those causal variables and the 
measure itself cannot be very large (Ahadi 
& Diener, 1989; Strube 1991). We expect 
many different neural parameters to influ -
ence broad personality traits such as the Big 
Five; hence, even relatively small effect sizes 
may indicate an important neural correlate 
that will help us to understand the biologi -
cal mechanisms underlying personality. Our 
recommendation is that sample sizes should 
be about 100 at a minimum, and preferably 
over 200 (Allen & De  Young, 2017). Note that such samples are still relatively small 
and would not be well powered to detect 
small effects, but we recognize the difficulty 
in funding larger studies, especially in neu -
roimaging.
One way to achieve larger samples for 
studying personality in neuroimaging is to 
include a standard set of personality mea -
sures, structural scans, and a brief period of 
resting-  state fMRI in smaller studies focused 
on within- rather than between-  subjects hy -
potheses (Mar, Spreng, & De  Young, 2013). 
When enough smaller studies have been 
completed, the measures in common across 
them can be aggregated into a dataset suffi -
cient for studying individual differences.
Statistical power in neuroimaging studies 
can be improved by focusing on regions or 
networks of interest rather than conducting 
whole-brain or whole-  cortex analyses. The 
studies and theories we have reviewed in this 
chapter provide many possibilities for the 
development of hypotheses regarding the as -
sociation of traits with specific brain regions 
and networks. The importance of focal hy -
potheses additionally highlights the impor -
tance of psychological theory for person -
ality neuroscience. Once one has a theory 
regarding the psychological functions that 
underlie personality traits, the way is open 
for development of neural hypotheses based 
on what is known about how the brain car -
ries out the functions in question (Allen & 
De Young, 2017). We recommend that re -
searchers studying individual differences in 
neural parameters familiarize themselves 
with theories of personality to guide them in 
formulating hypotheses.
Finally, we recommend that researchers 
keep in mind the hierarchical structure of 
personality traits. It is easy to focus on the 
Big Five given their prominence and the ex -
istence of relatively brief measures of them. 
However, different subfactors within a Big 
Five dimension may have different neural 
correlates, as seen most clearly in research 
on openness and intellect (De  Young et al., 
2009; Grazioplene et al., 2016). Researchers 
should make use of questionnaire measures 
designed to break down broad trait dimen -
sions into narrower subtraits. Not only will 
these somewhat longer measures provide 
more reliable measurements of the broad 
traits in question, but they will also enable 