198 II. BIOLOGICAL  FOuNDATIONS
the emotional salience of rewarding as well 
as threatening stimuli (Stillman, Van Bavel, 
& Cunningham, 2015). fMRI studies have 
found neural activity in response to reward -
ing or pleasant stimuli to be associated with 
extraversion in all of these regions; how -
ever, almost all of these have used samples 
smaller than 20, rendering their evidentiary 
value quite low. One somewhat larger study 
(N = 52), which increased reliability by fo -
cusing only on brain regions that responded 
consistently to monetary gains or losses in a 
retest over 2.5 years later, found that extra -
version predicted activity in the nucleus ac -
cumbens during anticipation of gaining $5 
(Wu, Samanez-  Larkin, Katovich, & Knut -
son, 2014).
The best supported finding in personality 
neuroscience is probably the association of 
extraversion with dopamine. At least nine 
studies have indicated that the effects of 
manipulating dopamine pharmacologically 
are moderated by extraversion (Chavanon, 
Wacker, & Stemmler, 2013; Depue, Luci -
ana, Arbisi, Collins, & Leon, 1994; Mueller 
et al., 2014; Rammsayer, 1998; Rammsayer, 
Netter, & Vogel, 1993; Wacker, Chavanon, 
& Stemmler, 2006; Wacker, Mueller, Piz -
zagalli, Hennig, & Stemmler, 2013; Wacker 
& Stemmler, 2006). One study showed that 
participants high in extraversion were more 
sensitive to the rewarding properties of a 
dopamine agonist (a drug that increases do -
paminergic function) than those low in ex -
traversion, such that they responded more 
energetically and experienced greater posi -
tive affect in response to stimuli that had 
previously been paired with the drug experi -
ence, relative to unpaired stimuli and rela -
tive to introverts (Depue & Fu, 2013).
The association of extraversion with do -
pamine has also been supported more in -
directly through a line of EEG research on 
an electrical waveform known as the re -
ward positivity  (confusingly, this waveform 
is more commonly known as “feedback- 
related negativity,” but Proudfit [2015] has 
convincingly demonstrated that it is really a 
positive-  going waveform related to reward). 
The reward positivity appears 200–350 mil -
liseconds after receiving feedback about an 
outcome and reflects dopaminergic signal -
ing in the dorsal ACC (Hauser et al., 2014). 
It spikes in response to better-  than-  expected outcomes and declines below baseline in re -
sponse to worse-than-  expected outcomes, a 
pattern that is also evident in the firing of 
many dopaminergic neurons in other spe -
cies (De  Young, 2013; Sambrook & Goslin, 
2015). Studies of both adults and children 
indicate that extraversion predicts the am -
plitude of the reward positivity (Bress & 
Hajcak, 2013; Cooper, Duke, Pickering, & 
Smillie, 2014; Kujawa et al., 2015; Lange, 
Leue, & Beauducel, 2012; Smillie, Cooper, 
& Pickering, 2011; Smillie et al., 2019).
Dopamine directly produces desire or 
“wanting” rather than pleasure or “liking” 
of rewards (Berridge, Robinson, & Aldridge, 
2009). Its release causes learning about what 
is desirable and motivates behavioral pursuit 
of rewards. However, people high in extra -
version also report increased enjoyment of 
rewards, reflected in their higher levels of 
positive emotionality. Pleasure in response 
to reward is governed by a neurotransmitter 
system involving endogenous opiates. The 
dopaminergic and opiate systems interact 
closely, especially in the nucleus accumbens 
(Peciña, Smith, & Berridge, 2006). When 
dopaminergic drugs, such as amphetamine, 
produce pleasure, this is due to the release 
of opiates, and when opiate-  mediated enjoy -
ment of rewards (e.g., food) increases desire 
for those rewards, this is because the dopa -
minergic system has been activated (Berridge 
et al., 2009; Jayaram-  Lindström, Wennberg, 
Hurd, & Franck, 2004). Nonetheless, these 
are two distinct neurotransmitter systems, 
which may help to explain the difference 
between the two aspects of extraversion: 
assertiveness and enthusiasm. Assertiveness 
(also called “agentic extraversion”) appears 
to be more purely tied to variation in desire 
rather than enjoyment given that it does not 
include descriptions of positive emotional -
ity and was more strongly associated with 
dopamine in some pharmacological studies 
than was enthusiasm (Mueller et al., 2014; 
Wacker et al., 2012). Enthusiasm, which en -
compasses positive emotionality as well as 
sociability, may reflect variation in opiates 
as well as dopamine, although evidence for 
this link is scarce. One study showed that 
social closeness, a good indicator of enthusi -
asm, moderated sensitivity to an opiate ma -
nipulation (Depue & Morrone-  Strupinsky, 
2005; De  Young et al., 2013).