I think we all have a basic understanding of the meaning of the words “extravert” and “introvert.” And if you’ve been on Truity for more than 2.5 seconds, then you probably know which one you are—and if you don’t, you can find out here.

Carl Jung used these terms to describe the manner in which people direct their energy. Extraverts get their energy from external activities, people, and events; introverts get their energy from ideas, memories, and “me time.” But other than the fact that this rings true for many people, is there any science to back up the existence of introverts and extraverts?

As it turns out, there is! Of course, like so many things in life, introversion and extraversion exist on a spectrum. Some introverts may be “more introverted” than others and thus require more time alone to recharge their batteries, and some extraverts might need more stimulation than others. But there still remain key general differences in the brains of introverts and extraverts.

1. Introversion is associated with increased activity in frontal lobe regions. In a 1999 study, researchers gave a Five Factor Inventory (similar to the Big Five) to the participants in order to gauge who were introverts and who were extraverts. The participants were then given positron emission tomography (PET) scans in order to measure their cerebral blood flow (CBF). No significance was found between introverts and extraverts in terms of whole brain blood flow, but CBF was significantly different in specific parts of the brain. Introverts had increased blood flow in the frontal lobes, while extraverts had lower blood flow in regions associated with behavioral inhibition.

2. Extraverts tend to be more susceptible to positive incentives, and that might have something to do with dopamine. Numerous studies have shown that extraversion is correlated to being more motivated by rewards. Some have hypothesized that this is due to a greater number of dopamine receptors in the midbrain. (Dopamine is a neurotransmitter associated with reward-seeking behavior. So more of certain dopamine receptors means more potential to anticipate and feel the rush of reward.)

3. Introverts have greater blood flow on a certain acetylcholine pathway in the brain while extraverts have more blood flowing on a dopamine pathway. In a study described in The Introvert Advantage by Marti Olsen Laney, PsyD, cerebral blood flow (CBF) was once again measured by PET. Certain neural pathways had greater activity in extraverts relative to introverts, and vice versa. Dr. Laney explains how the pathways that were more active in extraverts are associated with dopamine, while the pathways that were more active in introverts are associated with acetylcholine. Acetylcholine is essential to attention, learning, and memory, and at certain receptors, it can also serve to reinforce behaviors. Importantly, the dopamine pathway is shorter than the acetylcholine one—which could explain why many extraverts are able to respond faster under an onslaught of stimulation.

Now that I’ve presented these studies, I feel bound to note that correlation does not imply causation. Therefore, although the introverts in the first study had greater CBF in their frontal lobes, higher blood flow in the frontal lobes is not necessarily the cause of introverted characteristics in a person. Behaving like an introvert could increase the blood flow to the frontal lobes. Or maybe being an introvert and having greater CBF in the frontal lobes are both caused by a third unknown factor. That disclaimer aside, the research suggests that introversion and extraversion are linked to brain structure and chemistry.

The frontal lobe of the brain contains the primary motor cortex, which controls voluntary body movement. However, it is also involved in “higher thinking” and memory. “Higher thinking” includes tasks such as decision-making, predicting consequences of events, and determining similarities and differences. The memory function of the frontal lobe is involved with retaining memories associated with emotions.

So what does this all mean? That introverts seem to have higher blood flow—and thus more activity—in their frontal lobes? Well, many introverts report engaging in running monologues with themselves that may include reflecting on past events and thinking about or planning for the future. In contrast, extraverts had higher blood flow in the posterior insula, which is activated by interpretation of current sensory information. The differences in the activity in these areas for introverts and extraverts may be reflected by the former’s tendency toward inward focus and the latter’s toward outward focus.

The frontal lobe is also where you find most of the cerebral cortex’s dopamine-sensitive neurons. However, pre-reward (or reward-expectant) processing happens among the dopamine-sensitive neurons of the midbrain, and post-reward processing happens in the forebrain, which includes the frontal lobe. The evidence that introverts’ frontal lobes are more active fits with the idea that introverts may be more reflective on past events.

Additionally, due to the high concentration of dopamine-sensitive neurons in the frontal lobes, a lot of activity in the frontal lobes could explain why introverts seem to be more sensitive to dopamine—and thus more likely to be overwhelmed by excessive stimulation. Meanwhile, extraverts get their energy from exactly the outside stimuli that might overwhelm introverts. Another piece of this puzzle is that introverts and extraverts seem to have different numbers of different types of dopamine receptors, which means that they respond to stimuli and rewards differently.

Clearly, introversion and extraversion are not things that we’ve just made up. Our brains process information and situations differently at a cellular and chemical level. Maybe it’s just the neuroscience major in me, but I think that is pretty freaking cool.

Rachel Suppok
Rachel holds a B.S. in Neuroscience and usually a cup of coffee. She is an INTJ, but she is not a super-villain. Yet. Folow Rachel on Twitter @rsuppok.