You’re not imagining it. Your extroverted friend who seems so different from you? It’s because her brain is different.

Here’s a quick-and-dirty guide to some key differences between introverts’ and extroverts’ brains, according to science — and let me tell you, it explains a lot.

Introverts Are Sensitive to Dopamine

Why do extroverts like action, but introverts like calm?

It may have to do with two powerful chemicals found in the brain — dopamine and acetylcholine, “jolt juices” that hugely impact our behavior.

Dopamine gives us immediate, intense zaps of happiness when we act quickly, take risks, and seek novelty. Acetylcholine, on the other hand, also rewards us, but its effects are more subtle — it makes us relaxed, alert, and content.

One explanation for introversion vs. extroversion, according to Dr. Marti Olsen Laney in her 2002 book, The Introvert Advantage, is extroverts are less sensitive to dopamine, so they need more of it to feel happy. The more they talk, move, and socialize, the more they feel dopamine’s pleasant effects.

But when it comes to us “quiet ones,” too much dopamine can overstimulate us, writes Laney, like a kid with a tummy ache hopped up on too much sugar. On the other hand, when we read, concentrate, or use our minds in some way, we feel good because our brains release acetylcholine — a happiness bump so gentle that extroverts hardly register it.

This explains, on some level, why extroverts may seek new and exciting situations — as well as social opportunities — while introverts would rather stay home with a good book or hang out in a meaningful way with just one other person.

Introverts Prefer a Different Side of Their Nervous System

Another difference between introverts and extroverts has to do with our nervous systems. Everyone’s nervous system has two “sides” — the sympathetic side, which triggers the “fight, fright, or flight” response, and the parasympathetic side, which is responsible for “rest and digest” mode.

In other words, the sympathetic side is like hitting the gas pedal, while the parasympathetic side is like slamming on the brakes.

When your sympathetic system is activated, your body gears up for action. Adrenaline is released, glucose energizes muscles, and oxygen increases. Areas of your brain that control careful, measured thinking are turned off, although dopamine increases alertness in the back of your brain.

On the other hand, when you engage the parasympathetic side, your muscles relax, energy is stored like a squirrel preparing for winter, and food is metabolized. Acetylcholine increases alertness and blood flow to the front of your brain.

To be clear, extroverts and introverts use both sides of their nervous systems at different times. But which side do we introverts generally prefer? You’ve probably already guessed: According to Dr. Laney, we prefer the parasympathetic side, which slows and calms us.

Introverts Use the Long Acetylcholine Pathway

Ever wonder why, as an introvert, you are prone to overthinking? It may have to do with how we process stimuli differently than extroverts.

When information from the outside world — like someone’s voice or images on a computer screen — enters an extrovert’s brain, it travels a shorter pathway, Laney theorizes. It passes through “quick response” areas of the brain where taste, touch, sight, and sound are processed.

But for introverts, the pathway may be longer, traveling through many areas of the brain, including:

The right front insular , which is an area associated with empathy, self-reflection, and emotional meaning. This is also the area of the brain that notices errors.

, which is an area associated with empathy, self-reflection, and emotional meaning. This is also the area of the brain that notices errors. Broca’s area , which plans speech and activates self-talk.

, which plans speech and activates self-talk. The right and left front lobes , which select, plan, and choose ideas or actions. These areas also develop expectations and evaluate outcomes.

, which select, plan, and choose ideas or actions. These areas also develop expectations and evaluate outcomes. The left hippocampus, which stamps things as “personal” and stores long-term memories.

If Laney’s theory is correct, this means introverts process information more thoroughly than extroverts do. No wonder it can take us longer to put our thoughts into words, react, or make decisions!

Introverts Have More Gray Matter

Finally, a study published in the Journal of Neuroscience found that introverts had larger, thicker gray matter in their prefrontal cortex — the area of the brain associated with abstract thought and decision-making. Extroverts had thinner gray matter in that same area. This suggests that introverts may devote more neural resources to abstract thought, while extroverts tend to live in the moment more.

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What This Research Means

It means that as an introvert, you were probably born this way. Although you will grow and change over time — as we all do — you’ll always have a general preference for solitude and calm. (Read more about the science behind why you probably won’t stop being an introvert here.)

It doesn’t mean you’ll never enjoy a party or seek new and exciting experiences. Nor does it mean that extroverts will never enjoy the peace that comes with solitude — we still get to choose what we do. And, interestingly, there’s some evidence that our personalities change over time (for the better), including that fact that we all get more introverted as we get older.

Keep in mind that “introversion and extroversion are not black and white. No one is completely one way or another — we all must function at times on either side of the continuum,” writes Dr. Laney in The Hidden Gifts of the Introverted Child.

So those loud, rowdy extroverts? Cut them a little slack. It’s their brain!

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