The neuroscience behind extroversion and introversion: are we stuck with the personality traits we are given at birth?

By Caitlin Davies

Objective personality type tests were first developed during the first World War in order to identify soldiers who were most likely to suffer from nervous breakdowns during enemy bombardment (Gibby & Zickar, 2008). Since then, personality type tests have become more sophisticated and are now commonplace in workplaces, schools and many other institutions. The general aim of the tests within these institutions is establish what personality traits an individual has in order to identify their strengths and weaknesses and how they are likely to respond to certain situations. Since their initial introduction, several variations have been developed as a result of the introduction of new theories regarding personality types and updated research. The most well-regarded theories of personality types that people are most familiar with are the Myers-Briggs system and the Big Five system. 

The Myers-Briggs system is a lettered system that was developed by Katherine Cook Briggs and her daughter Isabel Myers Briggs and was first published in 1962 (The Myers Briggs Company, No date). The system differentiates people into 16 different personality types by categorising them using four scales e.g INFJ, ENTP, ISFP. These scales are extroversion-introversion, sensing-intuition, thinking-feeling and judging-perceiving (The Very Well Mind, 2020).  The Big Five system on the other hand was developed and built upon by a number of psychologists during the 20th century (Truity, 2019). This system has five main scales of personality traits and these are extraversion-introversion, openness, neuroticism, agreeableness and conscientiousness (Suttie for the Greater Good Magazine, 2018). In order to determine your personality type, both these tests make use of a range of statements and then ask the individual to decide how much or how little they agree with these statements.

A personality trait that both personality systems identify in individuals is whether that individual is extroverted or introverted. While the definitions of each have changed over time, the general consensus seems to be that extroverted humans thrive in scenarios where they are surrounded by people and leave feeling energised whereas introverts prefer smaller intimate gatherings with close friends and need time alone in order to feel recharged (Bennington-Castro for Gismodo, 2013). As a result, there is a growing collection of research on how certain personality traits can affect our lives and studies have started to identify correlations between certain personality traits and outcomes in life e.g conscientiousness and a longer life span (Roberts et al., 2007), (Friedman et al., 1993). However, the effects of introversion and extroversion specifically on our everyday lives has been highlighted by studies that suggest those individuals high in extroversion have better social relationships, are better at integrating in social situations and have better support networks, all factors associated with better health outcomes (Roberts et al., 2007), (Berkman et al., 2000).

But what causes us to be either an introvert or an extrovert? Some longitudinal studies suggest whether we are an introvert or an extrovert is an innate quality that can be detected in early childhood, even as early as birth (Olsen Laney, 2006). Research by Kagan and Snidman showed that four month old babies could be categorised into two temperamental profiles called inhibited temperament and uninhibited temperament based on their reactivity to novel situations, the temperament profiles being what we now refer to as introversion (inhibited) and extroversion (uninhibited) (Kagan & Snidman, 1991).  Additionally, it is believed that out of all the personality traits, extroversion-introversion is the most strongly influenced by hereditary factors and remains stable throughout life (Olsen Laney, 2006).

Additionally, on a neurological level the dopamine hypothesis describes individual differences between an introvert’s brain compared to an extrovert’s brain. The first set of research describes extroversion being linked with genetic polymorphisms in the dopamine D4 receptor (Benjamin et al., 1996) and dopamine D2 receptor (Smillie et al., 2010) which may affect dopaminergic neurotransmission (Wacker and Smillie, 2015). The second set of research within the dopamine hypothesis focuses on individual differences between extroverts and introverts in the activation of the prefrontal regions of the brain’s reward system (Wacker & Smillie, 2015).  In a study conducted by Wu et al. involving the famous  Monetary Incentive Delay Task, fMRI results show that left nucleus accumbens (NAcc) responses during anticipation of rewards positively correlate with extroversion scores, suggesting extroverts are more sensitive to rewards  (Wu et al., 2014).  However, one review conducting a meta-analysis of dopamine hypothesis and extroversion research found the evidence regarding dopamine relevant genes and volume of dopamine rich brain regions was highly mixed with some studies having reliability issues, although results from electroencephalograms and fMRIs measuring neural responses to reward were more convincing albeit some reproducibility issues (Wacker & Smillie, 2015). 

While it is likely that introversion and extroversion have a genetic component, the influence of life experiences should not be disregarded. Findings from identical twin studies show that environmental influences are a major factor in determining personality and can account for at least half the variance (Plomin and Daniels, 2011). Findings from Loehlin and Nichols’ twin study suggests that personality has around a 40% genetic variance and a 60% environmental variance (Loehlin & Nichols, 2012).

So, while it seems extroversion or introversion are the result of a complex interaction between both genetic and environmental causes, the previous research does not answer whether we can change personality traits. A longitudinal study conducted over 50 years by Damian et al reported that overall, the majority of personality traits do stay relatively stable, although some traits are more likely to change than others. But the resounding finding was that personality traits are not fixed and people remain malleable, especially as they mature. This was evident in the fact that many individuals were found to increase in emotional stability over time. (Damian et al., 2018).

Another researcher that agrees personality traits are flexible is Professor Brian Little who developed the free trait theory. Little argues that we may be able to temporarily switch to the other end of the scale of a personality trait, if it is in pursuit of something we love. For example, he stated that ‘introverts may temporarily act as extroverts in order to advance projects requiring expressions of enthusiastic assertiveness’ (Little, 2008). Little however, does make note that this may come at the cost of an individual’s well being so the individual should make sure of their own restorative resources (Little, 2008).

All in all, it seems that personality traits are flexible. The traits we are born with determine how we are likely to respond in certain situations, but they are not definitive of us. Our previous life experiences do have an influence on our behaviours and make us who we are today. But we also have a choice to temporarily switch to another personality type when pursuing something we love, as long as we do not pretend to be someone we are not for too long.

References:

Benjamin, J. et al. (1996) ‘Population and familial association between the D4 dopamine receptor gene and measures of Novelty Seeking’, Nature Genetics, 12(1), pp. 81–84. doi: 10.1038/ng0196-81.

Bennington-Castro, J. (2013) The Science of What Makes an Introvert and an Extrovert, io9. Available at: https://io9.gizmodo.com/the-science-behind-extroversion-and-introversion-1282059791 (Accessed: 2 March 2021).

Berkman, L. F. et al. (2000) ‘From social integration to health: Durkheim in the new millennium’, Social Science & Medicine (1982), 51(6), pp. 843–857. doi: 10.1016/s0277-9536(00)00065-4.

Damian, R. I. et al. (2018) ‘Sixteen Going on Sixty-Six: A Longitudinal Study of Personality Stability and Change across 50 Years’. PsyArXiv. doi: 10.31234/osf.io/dmq5g.

Friedman, H. S. et al. (1993) ‘Does Childhood Personality Predict Longevity?’, p. 10.

Gibby, R. E. and Zickar, M. J. (2008) ‘A history of the early days of personality testing in American industry: an obsession with adjustment’, History of Psychology, 11(3), pp. 164–184. doi: 10.1037/a0013041.

Kagan, J. and Snidman, N. (1991) ‘Temperamental Factors in Human Development’, The American psychologist, 46, pp. 856–62. doi: 10.1037/0003-066X.46.8.856.

Little, B. R. (2008) ‘Personal Projects and Free Traits: Personality and Motivation Reconsidered’, Social and Personality Psychology Compass, 2(3), pp. 1235–1254. doi: https://doi.org/10.1111/j.1751-9004.2008.00106.x.

Loehlin, J. C. and Nichols, R. C. (2012) Heredity, Environment, and Personality: A Study of 850 Sets of Twins. University of Texas Press.

Olsen Laney, M. (2006) The Hidden Gifts of the Introverted Child.

Plomin, R. and Daniels, D. (2011) ‘Why are children in the same family so different from one another?*’, International Journal of Epidemiology, 40(3), pp. 563–582. doi: 10.1093/ije/dyq148.

Roberts, B. W. et al. (2007) ‘The Power of Personality: The Comparative Validity of Personality Traits, Socioeconomic Status, and Cognitive Ability for Predicting Important Life Outcomes’, Perspectives on Psychological Science, 2(4), pp. 313–345. doi: 10.1111/j.1745-6916.2007.00047.x.

Smillie, L. D. et al. (2010) ‘Variation in DRD2 dopamine gene predicts Extraverted personality’, Neuroscience Letters, 468(3), pp. 234–237. doi: 10.1016/j.neulet.2009.10.095.

Suttie, J. (2018) Can Your Personality Change Over Your Lifetime?, Greater Good. Available at: https://greatergood.berkeley.edu/article/item/can_your_personality_change_over_your_lifetime (Accessed: 2 March 2021).

The Myers Briggs Company (No date) The history of the MBTI® assessment. Available at: https://eu.themyersbriggs.com/en/tools/MBTI/Myers-Briggs-history (Accessed: 1 March 2021).

The Very Well Mind (2020) Myers-Briggs Type Indicator: The 16 Personality Types, Verywell Mind. Available at: https://www.verywellmind.com/the-myers-briggs-type-indicator-2795583 (Accessed: 1 March 2021).

Truity (2019) Development of the Big Five Personality Model, Truity. Available at: https://www.truity.com/book/development-big-five-personality-model (Accessed: 1 March 2021).

Wacker, J. and Smillie, L. D. (2015) ‘Trait Extraversion and Dopamine Function’, Social and Personality Psychology Compass, 9(6), pp. 225–238. doi: https://doi.org/10.1111/spc3.12175.

Wu, C. C. et al. (2014) ‘Affective traits link to reliable neural markers of incentive anticipation’, NeuroImage, 84, pp. 279–289. doi: 10.1016/j.neuroimage.2013.08.055.

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