What is the difference between an introvert and an extravert? Various pop-culture answers may come to mind, but the neuropsychological underpinnings of what makes someone vibrantly excited in social settings and another quivering in dread is surprisingly unknown. One of the fundamental traits in personality psychology, extraversion is the “most stable core trait and a universal component in personality theory,” but the mechanisms underneath this component of personality are heavily debated (Lei et al., 2015).
An influential lens to understanding extraversion is Carl Jung’s proposition that extraverts and introverts react to external stimuli differently (Lucas, 2001). Jung understood the extravert to be one that has “immediate reactions to the objects and stimuli that he or she experiences” and thus is more dependent on the external environment (Lucas, 2001). On the other hand, introverts in Jungian psychology are “oriented toward the subjective feelings that an object or stimulus creates” (Lucas, 2001). In other words, Jung understands extraverts and introverts as possessing different approaches to processing stimuli, where the introvert is able to have more introspection and the extravert simply reacts to the changing environment. This could explain a lot of the difference in perceived energy between introverts and extraverts, but little neuropsychological evidence is available to explain the underlying stimuli, and thus this theory remains poorly measured.
Another lens to consider is Hans Eysenck’s arousal theory, where instead of differences in reaction to external stimuli, introverts and extraverts are thought to be fundamentally different in their general arousal. This is linked to the activity of the ascending reticular activation system (ARAS), where introverts are thought to have a “low threshold for ARAS activation” whereas extraverts are thought to have higher thresholds (Lucas, 2001). In essence, extraverts are hypothesized to exert more activity and assertive tendencies because they are seeking more arousal due to their lower internal arousal, and introverts are the opposite. However, studies of attention and memory tasks expose the limitations of arousal theory. Overall, there is a “failure of any study of performance to demonstrate directly that extraverts are in fact arousal-mediated” (Matthews and Gilliland, 1999). It does not explain “personality effects on performance in the afternoon and evening” nor situations like “demanding target detection tasks (Matthews and Gilliland, 1999). Thus, although this theory is plausible at first, it is also inconclusive in defining an underlying mechanism for extraversion.
Extraversion can also be considered a function of the reward mechanisms, initially proposed by Jeffrey Gray but later modified and extended by various researchers (Lucas, 2001). Although not immediately intuitive, the array of traits such as “assertiveness, sociability, and talkativeness” are thought to be directly related to sensitivity to reward (DeYoung, 2010). What this theory proposes is that extraverts and introverts engage with reward differently, where extraverts are more sensitive to reward while introverts are less sensitive. Conceptually, this would explain why extraversion often manifests in social behavior, assertiveness, and adventure seeking, as these are all behaviors associated with obtaining reward.
This theory is also supported by developments in modern neuroscience. The key brain regions in coding value and reward are the orbitofrontal cortex, striatum, and nucleus accumbens, as well as areas pertaining to emotion such as the amygdala. These areas are discovered to be related to reward independent of personality research. If this theory of reward-activated extraversion has biological grounding, then extraversion should be associated with structural and functional differences in these brain regions. Various studies have attempted to examine if such connections exist.
Taking a structural approach, a 2010 study attempted to see if volume of different brain regions varied consistently with this psychological theory (DeYoung, 2010). MRI scans were analyzed for local brain volume while controlling for other personality traits plus demographic characters of age, sex, and whole brain volume. They found that extraversion was significantly “associated with the volume of medial orbitofrontal cortex,” which is involved in coding “reward values of stimuli” (DeYoung, 2010). In other words, higher reported extraversion correlated with higher volumes of the medial orbitofrontal cortex. Moreover, a positive correlation of “orbital frontal cortex thickness with extraversion” has been established (Cremers et al., 2011). Functional evidence also converges to suggest the role of the orbitofrontal cortex and extraversion. For example, “extraversion also correlates with increased glucose metabolism in the orbitofrontal cortex” which suggests that extraverts are reacting to value judgements in the reward circuitry (Lei et al., 2015). Overall, there seems to be a strong relationship between the orbitofrontal cortex and extraversion, which is consistent with the theory that extraversion is fundamentally a manifestation of reward mechanisms.
In general, extraversion was associated with activation in the reward system structures. In task-related neuroimaging, where self-reported introverted and extraverted participants are presented with an n-back task or with the emotional Stroop task, “extraversion is associated with activations in regions of the posterior cingulate/precuneus (PCC), medial prefrontal cortex (mPFC), dorsolateral prefrontal cortex (dlPFC), anterior cingulate cortex (ACC), the amygdala, and the nucleus accumbens” (Lei et. al, 2015). Furthermore, resting-state neuroimaging has revealed that “spontaneous oscillations in the PCC, mPFC, orbitofrontal cortex, insula and hippocampus are significantly correlated with extraversion” (Lei et. al, 2015). Thus, there seems to be a deeper biological basis to extraversion and reward as resting state neuroimaging provides an understanding of the individual independent of experimental stimuli. The fact that these structures are consistently reward and decision oriented further supports the framework.
Finally, extraversion is connected on the neurotransmitter level with dopaminergic systems, which is further evidence of the theory as dopamine is understood to be heavily associated with reward. This is consistent with the above evidence as “both the wanting and liking traits are associated with the dopaminergic system” which are the “nucleus accumbens and amygdala, crucial regions in motivation, emotion, and reward” (Lei et al., 2015). The amygdala and nucleus accumbens were shown through EEGs, MRIs, and fMRIs to have associations both structurally and functionally with extraversion. Further, the orbitofrontal cortex is an area “densely innervated by mesolimbic dopamine fibers and implicated in coding the reward value of stimuli” and it is also shown to correlate strongly with extraversion (Wacker & Smillie, 2015). Specifically “despite considerable methodological variance between studies” extraversion has been repeatedly shown to “[modulate] various responses to dopaminergic drugs” (Wacker & Smillie, 2015). This provides consistent evidence through a different view of reward systems, through the neurotransmitter itself.
Altogether, the repeated neurological evidence both structurally and functionally on the association between reward systems and extraversion supports the reward-activation theory the most. Similar correlations were found across studies with diverse methodology and subject groups. However, the converging evidence is not to suggest there are no limitations or further work to be done. A fundamental limitation is that the subject groups of extraverts and introverts were mostly determined based on a self-reported questionnaire. These are also speculated theories to explain the observed behavioral differences between introverts and extraverts. As technology develops, methodology should be further explored to find more causal understandings of extraversion.
About the Author
Victoria Li is a freshman at Harvard College concentrating in Economics.
References
Cremers H, van Tol M-J, Roelofs K, Aleman A, Zitman FG, et al. (2011) Extraversion Is Linked to Volume of the Orbitofrontal Cortex and Amygdala. PLoS ONE 6(12): e28421. doi:10.1371/journal.pone.0028421
DeYoung, C. G., Hirsh, J. B., Shane, M. S., Papademetris, X., Rajeevan, N., & Gray, J. R. (2010). Testing predictions from personality neuroscience. Brain structure and the big five. Psychological science, 21(6), 820–828. https://doi.org/10.1177/0956797610370159
Lei, X., Yang, T., & Wu, T. (2015). Functional neuroimaging of extraversion-introversion. Neuroscience bulletin, 31(6), 663–675. https://doi.org/10.1007/s12264-015-1565-1
Lucas, R. E., & Diener, E. (2001). Extraversion. In N. J. Smelser & P. B. Baltes (Eds.), International Encyclopedia of the Social & Behavioral Sciences (pp. 5202-5205). Oxford: Pergamon.
Matthews, G., & Gilliland, K. (1999). The personality theories of H. J. Eysenck and J. A. Gray: A comparative review. Personality and Individual Differences, 26(4), 583–626. https://doi.org/10.1016/S0191-8869(98)00158-5
Smillie, L. D. (2013). Extraversion and reward processing. Current Directions in Psychological Science, 22(3), 167–172. https://doi.org/10.1177/0963721412470133
Wacker, & Smillie, L. D. (2015). Trait Extraversion and Dopamine Function. Social and Personality Psychology Compass, 9(6), 225–238. https://doi.org/10.1111/spc3.12175
An influential lens to understanding extraversion is Carl Jung’s proposition that extraverts and introverts react to external stimuli differently (Lucas, 2001). Jung understood the extravert to be one that has “immediate reactions to the objects and stimuli that he or she experiences” and thus is more dependent on the external environment (Lucas, 2001). On the other hand, introverts in Jungian psychology are “oriented toward the subjective feelings that an object or stimulus creates” (Lucas, 2001). In other words, Jung understands extraverts and introverts as possessing different approaches to processing stimuli, where the introvert is able to have more introspection and the extravert simply reacts to the changing environment. This could explain a lot of the difference in perceived energy between introverts and extraverts, but little neuropsychological evidence is available to explain the underlying stimuli, and thus this theory remains poorly measured.
Another lens to consider is Hans Eysenck’s arousal theory, where instead of differences in reaction to external stimuli, introverts and extraverts are thought to be fundamentally different in their general arousal. This is linked to the activity of the ascending reticular activation system (ARAS), where introverts are thought to have a “low threshold for ARAS activation” whereas extraverts are thought to have higher thresholds (Lucas, 2001). In essence, extraverts are hypothesized to exert more activity and assertive tendencies because they are seeking more arousal due to their lower internal arousal, and introverts are the opposite. However, studies of attention and memory tasks expose the limitations of arousal theory. Overall, there is a “failure of any study of performance to demonstrate directly that extraverts are in fact arousal-mediated” (Matthews and Gilliland, 1999). It does not explain “personality effects on performance in the afternoon and evening” nor situations like “demanding target detection tasks (Matthews and Gilliland, 1999). Thus, although this theory is plausible at first, it is also inconclusive in defining an underlying mechanism for extraversion.
Extraversion can also be considered a function of the reward mechanisms, initially proposed by Jeffrey Gray but later modified and extended by various researchers (Lucas, 2001). Although not immediately intuitive, the array of traits such as “assertiveness, sociability, and talkativeness” are thought to be directly related to sensitivity to reward (DeYoung, 2010). What this theory proposes is that extraverts and introverts engage with reward differently, where extraverts are more sensitive to reward while introverts are less sensitive. Conceptually, this would explain why extraversion often manifests in social behavior, assertiveness, and adventure seeking, as these are all behaviors associated with obtaining reward.
This theory is also supported by developments in modern neuroscience. The key brain regions in coding value and reward are the orbitofrontal cortex, striatum, and nucleus accumbens, as well as areas pertaining to emotion such as the amygdala. These areas are discovered to be related to reward independent of personality research. If this theory of reward-activated extraversion has biological grounding, then extraversion should be associated with structural and functional differences in these brain regions. Various studies have attempted to examine if such connections exist.
Taking a structural approach, a 2010 study attempted to see if volume of different brain regions varied consistently with this psychological theory (DeYoung, 2010). MRI scans were analyzed for local brain volume while controlling for other personality traits plus demographic characters of age, sex, and whole brain volume. They found that extraversion was significantly “associated with the volume of medial orbitofrontal cortex,” which is involved in coding “reward values of stimuli” (DeYoung, 2010). In other words, higher reported extraversion correlated with higher volumes of the medial orbitofrontal cortex. Moreover, a positive correlation of “orbital frontal cortex thickness with extraversion” has been established (Cremers et al., 2011). Functional evidence also converges to suggest the role of the orbitofrontal cortex and extraversion. For example, “extraversion also correlates with increased glucose metabolism in the orbitofrontal cortex” which suggests that extraverts are reacting to value judgements in the reward circuitry (Lei et al., 2015). Overall, there seems to be a strong relationship between the orbitofrontal cortex and extraversion, which is consistent with the theory that extraversion is fundamentally a manifestation of reward mechanisms.
In general, extraversion was associated with activation in the reward system structures. In task-related neuroimaging, where self-reported introverted and extraverted participants are presented with an n-back task or with the emotional Stroop task, “extraversion is associated with activations in regions of the posterior cingulate/precuneus (PCC), medial prefrontal cortex (mPFC), dorsolateral prefrontal cortex (dlPFC), anterior cingulate cortex (ACC), the amygdala, and the nucleus accumbens” (Lei et. al, 2015). Furthermore, resting-state neuroimaging has revealed that “spontaneous oscillations in the PCC, mPFC, orbitofrontal cortex, insula and hippocampus are significantly correlated with extraversion” (Lei et. al, 2015). Thus, there seems to be a deeper biological basis to extraversion and reward as resting state neuroimaging provides an understanding of the individual independent of experimental stimuli. The fact that these structures are consistently reward and decision oriented further supports the framework.
Finally, extraversion is connected on the neurotransmitter level with dopaminergic systems, which is further evidence of the theory as dopamine is understood to be heavily associated with reward. This is consistent with the above evidence as “both the wanting and liking traits are associated with the dopaminergic system” which are the “nucleus accumbens and amygdala, crucial regions in motivation, emotion, and reward” (Lei et al., 2015). The amygdala and nucleus accumbens were shown through EEGs, MRIs, and fMRIs to have associations both structurally and functionally with extraversion. Further, the orbitofrontal cortex is an area “densely innervated by mesolimbic dopamine fibers and implicated in coding the reward value of stimuli” and it is also shown to correlate strongly with extraversion (Wacker & Smillie, 2015). Specifically “despite considerable methodological variance between studies” extraversion has been repeatedly shown to “[modulate] various responses to dopaminergic drugs” (Wacker & Smillie, 2015). This provides consistent evidence through a different view of reward systems, through the neurotransmitter itself.
Altogether, the repeated neurological evidence both structurally and functionally on the association between reward systems and extraversion supports the reward-activation theory the most. Similar correlations were found across studies with diverse methodology and subject groups. However, the converging evidence is not to suggest there are no limitations or further work to be done. A fundamental limitation is that the subject groups of extraverts and introverts were mostly determined based on a self-reported questionnaire. These are also speculated theories to explain the observed behavioral differences between introverts and extraverts. As technology develops, methodology should be further explored to find more causal understandings of extraversion.
About the Author
Victoria Li is a freshman at Harvard College concentrating in Economics.
References
Cremers H, van Tol M-J, Roelofs K, Aleman A, Zitman FG, et al. (2011) Extraversion Is Linked to Volume of the Orbitofrontal Cortex and Amygdala. PLoS ONE 6(12): e28421. doi:10.1371/journal.pone.0028421
DeYoung, C. G., Hirsh, J. B., Shane, M. S., Papademetris, X., Rajeevan, N., & Gray, J. R. (2010). Testing predictions from personality neuroscience. Brain structure and the big five. Psychological science, 21(6), 820–828. https://doi.org/10.1177/0956797610370159
Lei, X., Yang, T., & Wu, T. (2015). Functional neuroimaging of extraversion-introversion. Neuroscience bulletin, 31(6), 663–675. https://doi.org/10.1007/s12264-015-1565-1
Lucas, R. E., & Diener, E. (2001). Extraversion. In N. J. Smelser & P. B. Baltes (Eds.), International Encyclopedia of the Social & Behavioral Sciences (pp. 5202-5205). Oxford: Pergamon.
Matthews, G., & Gilliland, K. (1999). The personality theories of H. J. Eysenck and J. A. Gray: A comparative review. Personality and Individual Differences, 26(4), 583–626. https://doi.org/10.1016/S0191-8869(98)00158-5
Smillie, L. D. (2013). Extraversion and reward processing. Current Directions in Psychological Science, 22(3), 167–172. https://doi.org/10.1177/0963721412470133
Wacker, & Smillie, L. D. (2015). Trait Extraversion and Dopamine Function. Social and Personality Psychology Compass, 9(6), 225–238. https://doi.org/10.1111/spc3.12175
