Are Introverts and Extroverts just built different?
The Biology, Neuroscience and Genetics behind extraversion (Part 2)
Introverts and extroverts are often painted as two very different types of people, almost even different species. But how true is this biologically?
In my last article, I wrote about what introversion and extroversion were and how the two camps differed. To summarize both terms are opposite sides of a psychological trait called extraversion.
The term describes people’s behaviour on a spectrum encompassing traits such as sociability, assertiveness, positive emotionality, approach tendencies, and status motivation. What differentiates introverts from extroverts is where their predispositions lie on these.
Now I want to analyze the biological differences between the 2 camps and see the scientific theories for what internal mechanisms drive them.
Bio-psychological Theories
When the preeminent psychologist Eysenck placed extraversion and other traits in his theory of personality he believed they varied due to biological differences rooted in the brain. Despite neuroscience not advancing to the level we have today psychologists still put out theories to explain these differences.
This began with Eysneck. He theorized that differences in extraversion lay in the natural arousal levels in the ascending reticular activation system (ARAS). The ARAS was a neural system that was found to be responsible for controlling the body’s attention/alertness/arousal.
Eysenck postulated that introverts had a higher resting arousal level while extroverts had a lower resting arousal level. Because introverts already had high levels of arousal they would be dissuaded from partaking in exciting activities to avoid overstimulation. Conversely, extroverts were incentivized to partake in more stimulating activities to raise their level of arousal.
This explains why extroverts tend to engage in more sociable activities and feel at home in busy environments. While introverts tended to engage in more reserved activities and feel more at home in calm environments.
Jeffrey Gray (a student of Eysneck), developed an alternative theory called Reinforcement Sensitivity Theory (RST). It was instead based on research analyzing animal responses to rewards and punishments.
Gray’s nervous framework consisted of three systems:
Fight-flight-freeze system (FFFS) : mediating reactions to negative stimuli
Behavioural approach system (BAS) : mediating reactions to rewarding stimuli
Behavioural inhibition system (BIS) : mediating conflicts between the BAS and FFFS, especially during uncertainty or goal conflict
He postulated that extroverts had a stronger BAS that inclined them towards goal-seeking behaviour. However, it also came with a risk of higher impulsivity and addictive behaviours. On the other hand, introverts had a stronger BIS causing them to be more behaviourally cautious. This came with the Achilles heel of being predisposed to anxiety and overthinking.
Over time it seems like Eysenck’s theory has fallen out of favour. Gray’s theory tends to be more tested and examined due to how appealing the reward processing system seems to be. Richard Depue and various contributors developed an offshoot of Gray’s model that theorized that extraversion could be related to differences in dopamine function across individuals.
Dopamine is a complex hormone and neurotransmitter involved in numerous body functions but is most known for its effect on causing feelings of pleasure, satisfaction and motivation. Depue et al theorized that extroverts have a more responsive dopamine system, causing an increased sensitivity to rewarding stimuli.
This heightened sensitivity motivates them to experience positive emotions more strongly and be more motivated to approach potentially rewarding scenarios. This model fits all previously mentioned differences between extroverts and introverts. More importantly to me, it also points to why extroverts tend to approach social and non-social rewards more often and more vigorously.
Despite how conclusive this may seem, the specifics are still being hashed out in the arena of neuroscience.
Neuroscientific findings
The brain is a notoriously easy organ to analyze. So easy in fact that many doctors claim neurosurgery to be the “easiest” medical specialty. So of course psychologists have had an equally easy time obtaining personality differences from neuroscience, right?
Wrong. Studies are still ongoing and widespread. Much of the field’s advancements are chronicled in the Personality Neuroscience Journal. It explores the neuroscience of personality with an emphasis on finding its underlying causes.
Numerous neuroscientific studies have been done to verify, debunk and expand on Gray’s, Eysenck’s, Depue’s and others’ theories. Some of the techniques used to analyze these include:
Magnetic Resonance Imaging (MRI): using magnetic and radio waves to create images of the inside of the brain
Electroencephalogram (EEG): measurement of electrical activity on the surface of the brain through the scalp to record changes in brain activity
Pharmacological Manipulation: altering levels of biochemicals and observing the effects on behaviour.
As earlier stated extraversion’s relation to dopamine seems to be the most promising area of study. It is one of the few theories that has had mild replication success with at least nine studies finding that the effects of dopaminergic drugs are moderated by one’s level of extraversion.
Going into specifics, neuroimaging studies showed that extraversion is positively associated with activity in certain dopaminergic-influenced brain areas during reward processing and working memory tasks.
Different brain regions have been found to be responsible for different facets of extraversion. One region was related to “agentic extraversion” which involves the more goal-oriented aspect of extraversion such as tendencies towards assertiveness, persistence, and achievement. The other region was related towards “affiliative extraversion” which involved the more social aspect of extraversion, namely: amiability, gregariousness, and affection.
Despite the encouraging findings from some researchers there still seems to be some replication issues with these and other findings. One meta-study on fMRI data found no correlations between any personality trait and neural activation. Another meta-analysis flatly found no brain differences as a function of the Big Five personality traits.
To cut the researchers some slack, the field is barely 20 years old. This is mainly due to how new the tools they are using to measure are. But more work needs to be done to find broad-based consistency.
While researchers are trying to find some consistency in extraversion from the brain, some are looking for it in our genes.
Genetic theories
Like our physical traits, many believe our behavioural ones are also inherited. This is theorized to occur due to evolution, with the aim of organisms to spread there genes.
Depending on the environment one personality type may be more beneficial than the other. In the case of extraversion, higher levels of extraversion have been linked to higher social dominance, more interpersonal relationships and more sexual partners. From an evolutionary perspective, these benefits would aid in spreading one’s lineage.
Conversely, extroverts tend to engage in more risky and thrill-seeking behaviour, exposing them to more danger and death. This would reduce the chance of them being able to pass on their genes. Introversion has been hypothesized to be a protective trait against infectious diseases. Those who kept to themselves more often would be less prone to contact illnesses during pandemics.
A meta-analysis examining genetics and extraversion could not find a gene associated directly with extraversion. However, they identified a long non-coding RNA (lncRNAs) site that did. Unlike regular genes that code for proteins, lncRNAs help regulate how other genes are turned on or off, potentially influencing personality traits. This suggests that extraversion may not be controlled by a single gene but rather by complex gene regulation mechanisms involving multiple genes.
Depending on the study researchers have estimated that 30-60% of personality is inheritable. Strong evidence of this came from studying families, especially twins and adopted family members. Researchers found that twins raised apart tended to still have similar personalities. In addition, children raised by adoptive parents tended to still show similar personalities to their biological ones. This suggests that personality traits like extraversion are often inheritable regardless of the environment.
Despite how much I and other researchers want to separate the influences of nature and nurture, it seems that genetic and environmental factors both interact in complex ways to form people’s personality.
Like many things in social science, very few findings are definitive. Despite that, it was an interesting experience to go through the advancements made in the field and see where things were going.
One thing that I kept wondering after reviewing all this info was, how these researchers categorize introverts and extroverts before trying to find the biological differences between the groups.
Unlike the internal biological probes, they had to take an external observational metric. How is that measured? How exact is it? How different are introverts and extroverts? Is a low-level introvert barely different from a low-level extrovert?
I’ll address these and other questions in the next instalment of this deep dive into the introvert-extrovert debate.
References
Bio-psychological Theories
[1] 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
[2] Rusting, C. L., & Larsen, R. J. (1997). Extraversion, neuroticism, and susceptibility to positive and negative affect: A test of two theoretical models. Personality and Individual Differences, 22(5), 607–612. https://doi.org/10.1016/s0191-8869(96)00246-2
[3] Walker, D. L. (2020). Extraversion – Introversion. The Wiley Encyclopedia of Personality and Individual Differences, 1, 159–163. https://doi.org/10.1002/9781119547143.ch28
[4] Wilt, J., & Revelle, W. (2016). Extraversion. In T. A. Widiger (Ed.), Oxford Handbooks Online (pp. 57–82). Oxford University Press. https://doi.org/10.1093/oxfordhb/9780199352487.013.15
[5] Smillie, L. D. (2013). Extraversion and Reward Processing. Current Directions in Psychological Science, 22(3), 167–172. https://doi.org/10.1177/0963721412470133
[6] Matthews, G. (2019). Extraversion-Introversion. Reference Module in Neuroscience and Biobehavioral Psychology, 1. https://doi.org/10.1016/b978-0-12-809324-5.21765-3
[7] Pickering, A., Cooper, A., Smallie, L., & Corr, P. (2013, January 16). On the shoulders of giants. BPS; the british psychological society. https://www.bps.org.uk/psychologist/shoulders-giants
[8] Matthews, G. (2016). Traits, cognitive processes and adaptation: An elegy for Hans Eysenck’s personality theory. Personality and Individual Differences, 103, 61–67. https://doi.org/10.1016/j.paid.2016.04.037
Neuroscience
[9] DeYoung, C. G., Beaty, R. E., Genç, E., Latzman, R. D., Passamonti, L., Servaas, M. N., Shackman, A. J., Smillie, L. D., Spreng, R. N., Viding, E., & Wacker, J. (2022). Personality Neuroscience: An Emerging Field With Bright Prospects. Personality Science, 3, 1–21. https://doi.org/10.5964/ps.7269
[10] Matthews, G., & Corr, P. J. (2009). The Cambridge handbook of personality psychology (pp. 331–332). Cambridge University Press.
[4] Wilt, J., & Revelle, W. (2017). Extraversion. In T. A. Widiger (Ed.), The Oxford handbook of the Five Factor Model (pp. 57–81). Oxford University Press. https://doi.org/10.1093/oxfordhb/9780199352487.013.15
[12] DeYoung, C. G. (2010). Personality Neuroscience and the Biology of Traits. Social and Personality Psychology Compass, 4(12), 1165–1180. https://doi.org/10.1111/j.1751-9004.2010.00327.x
[13] Hyatt CS, Hallowell ES, Owens MM, Weiss BM, Sweet LH, and Miller JD. (2020) An fMRI investigation of the relations between Extraversion, internalizing psychopathology, and neural activation following reward receipt in the Human Connectome Project sample. Personality Neuroscience. Vol 3: e13, 1-15. doi: 10.1017/ pen.2020.11
[14] Chen Y-W and Canli T. (2022) “Nothing to see here”: No structural brain differences as a function of the Big Five personality traits from a systematic review and meta-analysis. Personality Neuroscience. Vol 5: e8, 1–28. doi: 10.1017/pen.2021.5
[15] Corr, P. J., & Mobbs, D. (2018). From Epiphenomenon to Biologically Important Phenomena. Personality Neuroscience, 1. https://doi.org/10.1017/pen.2017.1
[16] Chen, Y.-W., & Canli, T. (2022). “Nothing to see here”: No structural brain differences as a function of the Big Five personality traits from a systematic review and meta-analysis. Personality Neuroscience, 5. https://doi.org/10.1017/pen.2021.5
Genes
[17] Zwir, I., Arnedo, J., & Del-Val, C. (2018). Uncovering the complex genetics of human character. Molecular Psychiatry, 25(10), 2295–2312. https://doi.org/10.1038/s41380-018-0263-6
[18] Hullett, A. (2024, September 18). The Science of Self: How Much of Your Personality Is Encoded in Your DNA? Healthline; Healthline Media. https://www.healthline.com/health/mental-health/is-your-personality-genetic
[4] Wilt, J., & Revelle, W. (2016). Extraversion. In T. A. Widiger (Ed.), Oxford Handbooks Online (pp. 57–82). Oxford University Press. https://doi.org/10.1093/oxfordhb/9780199352487.013.15
[20] Kandler, C. (2012). Nature and Nurture in Personality Development. Current Directions in Psychological Science, 21(5), 290–296. https://doi.org/10.1177/0963721412452557
[21] Vukasović, T., & Bratko, D. (2015). Heritability of personality: A meta-analysis of behavior genetic studies. Psychological Bulletin, 141(4), 769–785. https://doi.org/10.1037/bul0000017
[22] van den Berg, S. M., de Moor, M. H. M., Verweij, K. J. H., Krueger, R. F., Luciano, M., Arias Vasquez, A., Matteson, L. K., Derringer, J., Esko, T., Amin, N., Gordon, S. D., Hansell, N. K., Hart, A. B., Seppälä, I., Huffman, J. E., Konte, B., Lahti, J., Lee, M., Miller, M., & Nutile, T. (2015). Meta-analysis of Genome-Wide Association Studies for Extraversion: Findings from the Genetics of Personality Consortium. Behavior Genetics, 46(2), 170–182. https://doi.org/10.1007/s10519-015-9735-5