What makes someone become an introvert? It's a question that many an exasperated extrovert has asked of their introverted friends or coworkers — and one that many introverts have occasionally asked themselves (typically when we find ourselves trapped at a loud dance club, professional networking event, or other social occasion that sends us looking for a dark corner to scurry into). For most of the near-hundred years since pioneering psychiatrist Carl Jung coined the terms "introvert" and "extrovert," no one has been totally sure what makes some people love constant social activity and others run from it. But recently, science has begun doing some investigation on this front — and their results suggest that introverts and extroverts are born, not made.

Though many people confuse "introversion" with being shy and "extroversion" with being friendly or popular, the terms actually refer to your relationship with social interaction. Extroverts find social interaction emotionally nourishing and "recharging," while introverts find it taxing and often have to "recharge" after hanging out with friends or going to parties. Introversion and extroversion aren't whole personalities in and of themselves — rather, they're elements, but important ones. They also aren't the only two options — there's a whole spectrum of introversion and extroversion, and almost no one is a pure introvert or pure extrovert.

So what causes some of us to be stimulated by social interaction, while others find the prospect of making small talk with strangers as appealing as a case of food poisoning? The answer has long been believed to lie in things like formative psychological experiences. Those most likely still play a role, but recent research has made the case that most of us are born with a predisposition one way or the other — and it's built into our very brain wiring. What are the biological differences between introverted and extroverted people? Let's take a look.

How Introverts' Brains Are Different Than Extroverts' Brains

Introverts Aren't As Motivated By Novelty

One theory of the brain science behind introversion and extroversion suggests that it all comes down to dopamine. In 2005, researchers at the University of Amsterdam studied groups of volunteers who were identified as introverts and extroverts via a personality quiz. The volunteers gambled while researchers monitored the activity in two regions of their brains: the amygdala and the nucleus accumbens, which are both tied to excitement and reward. The amygdala handles emotional reactions, while the nucleus accumbens is tied to how we process dopamine, a chemical that we use to process "rewards" and positive reactions.

The researchers found that the folks identified as extroverts had stronger reactions in those two regions while gambling, implying that extroverts may actually have brain wiring that rewards seeking out novel activities like meeting new people, trying new things, and other hallmarks of extroverted behavior. Conversely, introverted brains may not reward such behaviors, which is why introverts might find staying home with a book more rewarding than going out to a club. They're literally not getting the same high from it that their extroverted companions are; they're just getting vodka tonics spilled all over their shoes.

Introverts Don't Care As Much About Human Faces

But maybe dopamine isn't enough to convince you that there might be inborn differences between introverts and extroverts. "Dopamine, schmopamine!" you shout, wherever you are (if you're an introvert, probably at home; if you're an extrovert, probably leading a 100-person conga line through the center of your town). And it's true that you can't build a case on dopamine alone. But luckily, we also have the fact that introverted brains basically don't care about human faces any more than they do flowers.

A 2010 study at the Salk Institute for Biological Sciences gathered together a group of subjects who fell all over the introversion-extroversion spectrum, from hardcore introverts and extroverts to folks who rated somewhere in the middle. The researchers then monitored the subjects' brain reactions as they were exposed first to a series of images of flowers and then a series of human faces, looking for a reaction called the "P300." The P300 (so named because it occurs within 300 milliseconds of being exposed to new stimuli) is an unconscious reaction the human brain has when it encounters new, sudden information, such as an unfamiliar image or a loud sound.

The tests found that extroverted subjects had much stronger P300 reactions to human faces, demonstrating a much sharper jump when shown a new human face than when they were shown a new flower. Conversely, introverts had the same P300 reactions when shown the faces as they did when shown the flowers. This implies that random faces (and the humans attached to them) have as much innate value to introverts as flowers. Which totally checks out for me, personally. But before you go telling your most introverted friend that she loves her garden more than you, please keep in mind that it's just one test.

Introverts Love To Plan

A 1999 study found that even the patterns of blood flow inside introverted and extroverted brains are different. The brains of introverts had greater blood flow through the frontal lobes and the anterior thalamus — areas associated with planning ahead, solving problems, remembering the past, and other fun things you can do alone in a room by yourself. Meanwhile, the brains of extroverts tended to focus blood in the posterior thalamus, the anterior cingulate gyrus, and the temporal lobes — areas that help us interpret the data that our senses take in from the outside world. Which means not only that introverted and extroverted brains prioritize different functions, but also that you can blame your brain the next time you get busted for not interpreting the data your senses take in from the outside world (aka spacing out).

So, Are Introverts Born Or Made?

Our brain responses can be shaped by the things we're exposed to early in life, so these studies don't necessarily make a perfect argument for the idea that we're born introverted or extroverted. But some research on genetics may suggest that we are.

There are certain genes linked to being extra-responsive to dopamine. The same University of Amsterdam study found that participants who had that dopamine gene were the same people who had increased brain activity while gambling. So a tendency toward introverted or extroverted behavior may not just be present in our brains; it might actually be encoded in our genes. Genes that allow us to react more strongly to dopamine may shape our personalities, leading us to seek out experiences more likely to release dopamine (or to avoid experiences that don't do anything for our extremely un-dopaminey brains).

As cool as all of this research is, of course, none of it has definitively proven that introversion and extroversion are completely inborn. Larger studies will need to be conducted, and the answer will most likely be something similar to current takes on the genetic causes of things like depression and anxiety. We can be born with predispositions, but life experiences often influence or cement our actual personalities.

But this research is promising — especially for introverts, who are so often told to "snap out of it" by people who assume that being introverted is a choice, or worse, simply the sign of a bad attitude. We exist in a society not created for the happiness or ease of the introvert, and it will probably take a long time for that to change. But if nothing else, this research might help your friends understand why you spent so much of the last office Christmas party hiding in the copier room.

Images: Daria Nepriakhina/ Unsplash, Giphy (4)