208 II. BIOLOGICAL  FOuNDATIONS
investigation of multiple levels of the trait 
hierarchy. We suspect that the 10 aspect- 
level traits will be particularly important for 
neuroscience research based on the Big Five 
given that they are likely to reflect the most 
important distinctions for discriminant va -
lidity within each of the Big Five (De  Young, 
2015; De  Young et al., 2007, 2016).
Personality neuroscience is of interest, in 
part, as basic science. A complete under -
standing of personality must include the 
biological mechanisms that instantiate it. 
But it is not merely basic science. Neuro -
biological knowledge can usefully constrain 
the development of psychological theory. 
Although one might explain personality in 
purely psychological terms, ensuring that 
one’s psychological explanations are com -
patible with brain function is likely to in -
crease their accuracy. The neuroscientific 
research linking extraversion to the brain’s 
reward system, for example, provides a cru -
cial piece of evidence for the reward sensitiv -
ity theory of extraversion. Further, with the 
increasing recognition that psychopathology 
is on a continuum with normal personality 
and shares a very similar covariance struc -
ture (De  Young et al., 2016; Krueger & Mar -
kon, 2014; Markon et al., 2005; Wright & 
Simms, 2015), personality theory is becom -
ing increasingly important for cutting-  edge 
neuroscientific research on psychopathol -
ogy (Abram & De  Young, 2017; Shackman 
et al., 2016). Understanding the mechanisms 
that the produce normal personality func -
tion will enable us to better understand how 
those mechanisms become dysfunctional in 
mental illness, which may in turn lead to 
better strategies for prevention, diagnosis, 
and treatment. We predict that, over the 
next couple decades, personality neurosci -
ence will develop from its current fledgling 
state into a discipline of central importance 
in psychology.
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