Individual Differences in the Brain: Extraversion

In 1967, Hans Eysenck proposed a theory of personality that changed the way individual differences in personality were viewed. For the first time, a theory was proposed in which the neurophysiological causes of personality were described. In his theory, Eysenck focused on explaining how individuals differ from each other in terms of extraversion and neuroticism. Basically what Eysenck proposed was the following: individuals differ in the way their central nervous system (a.k.a. the brain and the spinal cord) functions. He believed that the brains of individuals who are more introverted, respond differently to the same stimuli as compared to the brains of individuals who are more extraverted. Similarly, the brains of individuals with lower levels of neuroticism, respond differently to the same stimuli than the brains of those with higher levels.

Eysenck’s Big Three

Of course, Eysenck did not “just” say that there are individual differences in the functioning of the brain. He specified how he believed these differences came about. In his theory of personality, Eysenck distinguished three dimensions (rather than our famous Big Five personality traits). His Big Three consisted of extraversion, neuroticism and psychoticism. Because it is too much to discuss these three traits all in one post only, we will dedicate a post to each personality dimension in the following weeks. Today, we start with extraversion.

Eysenck defined the extraversion dimension as a continuum, ranging from introversion to extraversion. Individuals high on extraversion (“extraverts”) are characterized by positive affectivity, engagement with the external world, high sociability, energy, and assertiveness. Individuals low on extraversion (“introverts”) on the other hand are characterized by engagement with their inner world, less sociable behaviour, and more quiet behaviour. Note that the description of Eysenck’s extraversion dimension is very similar to that of the Big Five personality trait.

The extraverted vs. introverted brain

Thus, how do “extraverted” brains differ in functioning from “introverted” brains? According to Eysenck, the differences are caused by variability in cortical arousal (a.k.a. activation of the central and autonomic nervous system. Cortical arousal increases among other things wakefulness, vigilance and heart rate). He believed that individuals can have lower and higher response thresholds for cortical arousal. More introverted individuals would have lower thresholds, and more extraverted individuals would have higher thresholds. Having lower thresholds for cortical arousal means that an individual’s brain gets more aroused. Vice versa, having higher thresholds means that the individual’s brain gets less cortically aroused.

Okay.. so the level of cortical activation differs per individual

But what does that mean for daily life? Perhaps an example will make this more clear: Ask anybody to name the number one difference between an extravert and an introvert. Likely this person will reply with: extraverts want to have people around, and introverts prefer to be alone. Of course, reality is not as black-and-white as this answer, but there is some truth in it. Individuals who are more introverted tend to spend more time on their own as compared to individuals who are more extraverted. Why? Well, simply put: because their brains need to rest more.

Remember that according to Eysenck, introverted individuals have lower response thresholds for cortical arousal. This means that the introverted brain “responds” to more stimuli than the extraverted brain. Put an introvert and an extravert in the very same room, and the introverted brain will likely “react” more than the extraverted brain: it has more input to process. Therefore, the introverted brain will need to reload sooner than the extraverted brain, preferably in a place or situation with low stimuli input. The extraverted brain on the other hand is not tired at all yet and needs more stimuli input to get aroused. A great way of getting this input is by inviting people over or visiting busy places!

Where do you work?

This example also hints to something that Eysenck refers to as the U-shaped relation between cognitive performance and level of cognitive arousal. It means that cognitive performance is expected to be best when the brain is optimally aroused. Eysenck believed that when the brain is under-aroused or over-aroused, cognitive performance lowers. Therefore, more introverted individuals should perform better in quiet settings, and extraverted individuals better in noisier settings. This is all because of the amount of input their brains prefer to process. Introverted brains want less input to perform optimally, and extraverted brains prefer more.
Have you ever wondered why some people can only concentrate when it is completely silent, like in a library? And why others always listen to music loudly while working? Turns out, the answer might be all about response thresholds in the brain.

Does the neuropsychological perspective make sense?

In the last years, much neurological research has been conducted to examine Eysenck’s theory. In the review by Mitchell and Kumari (2016), fMRI, MRI, and connectivity (diffusion tensor imaging: DTI) studies from the last 15 years have been summarized. These studies showed support for many of Eysenck’s predictions, especially predictions regarding structural and functional correlates of personality on the neurological level. For instance, an individual’s level of extraversion appears to be positively correlated with the volume of his amygdala, orbitofrontal cortex, and other prefrontal cortex brain regions. This is remarkable, as affective disorders are often linked to impaired working of the amygdala and orbitofrontal cortex. It would therefore appear to be the case that higher levels of extraversion can protect an individual from developing an affective disorder.

Was Eysenck right regarding extraversion?

However, these findings tell us nothing about our main concern here: the relationship between extraversion and cortical arousal. Surprisingly, not much research has been done regarding this relationship specifically. As far as known, the only study that examined this relationship stems from 2004. In their fMRI study, Kumari, ffytche, Williams and Gray investigated the predictions concerning extraversion and cortical arousal at different levels of stimulation. In line with the U-shaped relation between cognitive performance and the level of cognitive arousal, introverts should outperform extraverts when the level of stimulation is low. Vice versa, extraverts should outperform introverts when the level of stimulation is high.

Importantly, the results of Kumari and colleagues regarding extraversion and cortical arousal turned out to be consistent with Eysenck’s proposed model. As these findings are very encouraging for the neuropsychological field to keep examining this topic, it is clear that more research is needed!

What do you think about Eysenck’s theory? Ever thought about personality differences on a neurological level? Is there more that you would like to examine in the neuropsychological field?


If you are interested in reading the literature used for this post, references and links are provided here:

Eysenck, H. (2006). The Biological Basis of Personality. New York: Routledge. https://www.taylorfrancis.com/books/9781351305273

Kumari, V., ffytche, D., Williams, S. C., & Gray, J. A. (2004). Personality predicts brain responses to cognitive demands. Journal of Neuroscience24(47), 10636-10641.

Mitchell, R. L., & Kumari, V. (2016). Hans Eysenck’s interface between the brain and personality: Modern evidence on the cognitive neuroscience of personality. Personality and Individual Differences103, 74-81.

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