Introverts & Extroverts: It’s Not as Simple as Shy or Outgoing (Part 2)

Originally published on the Michigan Science Writers

Key words: neuroscience, neurology, introverts, extroverts, personality, dopamine, acetylcholine

In my last post, I wrote about the social differences between introverts and extroverts and the misconceptions surrounding the two personalities. This post will focus on the underlying brain biology that contributes to whether a person is an extrovert or an introvert. 

The more I read about these personalities, the more I wondered—are there ways in which the biology can explain the social differences? It turns out that there are several known, key differences in the brain biology between introverts and extroverts.

Brains of Introverts and Extroverts Respond Differently to Social Stimuli

The first difference is the overall amount of blood that flows to the brain. A group measured the amount of blood flowing through the brain, the cerebral blood flow (CBF), of 50 participants who took the Eysenck Personality Inventory to assess their introversion or extroversion. The researchers found a statistically significant inverse correlation between the level of extroversion and the CBF, meaning that introverts have a significantly higher CBF than extroverts do.

Since blood flow to an area of the body suggests its activation, higher CBF in introverts suggests that introverts are more prone to cognitive stimulation than extroverts. This led researchers to believe that introverts’ brains are more prone to over-stimulation, so introverts may avoid over-stimulating environments, such as meeting new people. On the contrary, extroverts likely compensate for their lower CBF by seeking out stimulating, new, adrenaline-inducing experiences.

Secondly, the two personality types are associated with activation of different brain regions. Using a technique called positron emission tomography, researchers measured the blood flow of the subject through different regions of the brain and correlated this regional blood flow to the patient’s personality type.

Introversion was associated with increased blood flow to the frontal lobes and anterior thalamus. The frontal lobe, shown in Figure 1 below, is one of many regions of the brain involved in a functional loop called the “behavioral inhibition system,” which has previously been theorized to be more active in introverts. On the contrary, extroversion was associated with increased blood flow in the temporal lobes and posterior thalamus, which have been thought to be involved in an individual seeking out sensory and emotional stimulation.


Fig 1: Introverts and Extroverts preferentially activate different regions of the brain, such as the frontal and temporal lobes.  Image Credit: Google

The third major biological difference is the way in which introverts and extroverts respond to neurotransmitters, which are chemicals that communicate information in our brain. In his book “The Introvert Advantage,” Dr. Marti Olsen Laney describes why introverts and extroverts tend to gravitate toward more solitary and outgoing activities, respectively. This has to do with the d4dr gene, which encodes a dopamine receptor. Dopamine is a stimulating neurotransmitter that plays a key role in a person's response to reward and pleasure—more dopamine, the greater feeling of the reward.

Extroverts tend to have a longer d4dr gene, and make less dopamine at baseline. In turn, they rely on activities that produce more dopamine and adrenaline, another stimulating neurotransmitter, to compensate for their inherently low level of dopamine production. Hence, extroverts tend to feel better when they are out doing things, meeting people, and receiving a lot of outside stimulation. Therefore, the extrovert’s inherently low level of dopamine production means that they require outgoing, high-stimulation activities to compensate and activate the sympathetic nervous system. Barry Kaufman, the Scientific Director at the Positive Psychology Center of the University of Pennsylvania, explained that extroverts have a much more highly active dopamine system.

On the other hand, introverts have a shorter d4dr gene, and produce more dopamine at baseline. This makes it much easier for them to feel over-stimulated, especially in social and similar high-stimulation situations (Laney, 2002). They rely on another neurotransmitter—acetylcholine—to balance out the feeling of over-stimulation. Acetylcholine plays important roles in attention, learning, memory, and is important in sustaining a calming effect. This slows the body down so that introverts can process through their thoughts and emotions, and mull over things as necessary. The introvert’s preference toward acetylcholine implies that they’re biologically predisposed toward preferring laidback, resting activities.

Response to Non-Social Stimuli

Overall, introverts are more susceptible to over-stimulation since they inherently produce more dopamine, so they do not require stimulating activities like meeting people or doing things to get the “high” that comes from this hormone. In fact, they require a neurotransmitter with an opposing effect to prevent them from feeling overwhelmed. Conversely, extroverts produce less of dopamine, so they require doing and experiencing new things and meeting new people.

But do introverts and extroverts only react differently to social stimulation or for other stimuli as well? One group sought to answer the question of whether this neurological phenomenon was specific to all reward-based stimuli, or if social stimuli are processed differently in introverts and extroverts. They tested what they called the "allocation of attention," an overarching measure of brain response after showing subjects social stimuli (pictures of male and female faces) and non-social stimuli (pictures of purple and yellow flowers).

There was a significant, positive association between amount of extroversion and level of allocation of attention to social stimuli—meaning that extroverts allocate more attention to social stimuli than introverts do. On the other hand, introverts allocate more attention to non-social stimuli than social stimuli. What could explain this difference? The researchers hypothesize that social stimuli carry more motivational significance for extroverts, so extroverts pay more attention to social stimuli than introverts do.

The Neuroscience of MY Personality

As mentioned previously, I identify myself as an introvert. I affectionately refer to my inability to turn my brain off as analysis paralysis, and now I can explain that with the increased blood flow to my brain. In social situations, when I find myself very suddenly going from having a great time to having a desire to leave, I now know that the abundance of stimulation around me has led to an imbalance of dopamine and acetylcholine, leaving me feeling overwhelmed. As soon as I remove myself from the situation, and return to the comfort and calm of my apartment, I recover because my neurotransmitters are back in balance, and acetylcholine saves the day!

Though it is definitely evident that biology is affecting behavior in these two personality types, it is also likely that other factors are playing a role, such as person’s up-bringing, experiences, or environment, especially from a social and personal relationship perspective. But that is a discussion for another time. For now, I’m sure there are many introverts out there that find it reassuring to know that their day-to-day experiences actually have a biological basis. I can’t tell you how reassuring it is to have a biological understanding of some of my more unexplainable behaviors. I’m sure my introverted and extroverted friends feel that same way. Science for the win!

Further References:

Laney, M. O. (2002). The Introvert Advantage: How Quiet People Can Thrive in an Extrovert World. Workman Publishing Company, Inc.