Introvert or extrovert: the quiet brain and the loud brain

What science really says about introverts and extroverts and about the myths in between

At a party, two people stand at the edge of the room. One of them is talking brightly to everyone who passes, drifting from cluster to cluster, and seems to charge up as the evening grows louder. The other hangs back a little, watching, happier to talk for a long while with one person than briefly with ten, and after a couple of hours feels a deep, quiet pull towards home, and being alone.

Most people would say the first is an extrovert and the second an introvert. And that is roughly right. But what do those words actually mean? And, more importantly, what does science really say about the difference between these two kinds of people?

I should be honest with you: I am a pronounced introvert myself. An observer. Someone who would rather watch and listen than fill a room with words. And that is precisely why I wanted to know whether the stories that circulate about introverts and extroverts are actually true. Because a great deal is said about them that sounds lovely but simply isn’t so.

Let’s look at what we really know.

Where the words come from

The terms “introvert” and “extrovert” were made famous in the early twentieth century by the Swiss psychiatrist Carl Jung, in his work Psychological Types (1921).¹ Jung saw it as a difference in the direction in which a person’s attention and inner energy flow: the extrovert turns outward, towards the world and other people; the introvert turns inward, towards their own thoughts and their inner world.

It’s worth clearing up one misunderstanding straight away: introverted does not mean “shy”, and extroverted does not mean “self-assured”. You can be a confident introvert who gives a perfectly good presentation and is then utterly worn out. And you can be a shy extrovert who needs people but is frightened of rejection. Shyness is about fear. Introversion is about energy. Those are two very different things.

A volume dial in your head

In the 1950s and ’60s, the British psychologist Hans Eysenck carried out work that still underpins how we understand this today.² His idea, in plain language: imagine there is a kind of volume dial in your head for stimulation, sounds, bustle, conversation, impressions. In each of us, that dial is set a little differently.

In the introvert, the volume dial naturally sits rather high already. The brain is fairly stimulated even when not much is happening. That’s why an introvert becomes overstimulated more quickly by a busy party, loud music, or a long day full of people. It isn’t too little. It very soon becomes too much. The introvert seeks out quiet to bring the stimulation back down to a comfortable level.

In the extrovert, the volume dial naturally sits low. The brain needs more stimulation to reach a pleasant, alert level. That’s why an extrovert actively seeks out bustle, company, new experiences, to turn the stimulation up to a comfortable level. Silence and solitude can quickly feel, to the extrovert, like boredom or emptiness.

This picture explains much of what we see from the outside. The introvert who is exhausted after a party isn’t antisocial. The dial was simply turned up too high for too long. The extrovert who can’t sit still isn’t shallow. The dial is crying out for more input.

Eysenck placed that dial in a network that, from the brainstem, governs how awake and alert you are: the reticular activating system. You needn’t remember that name. Remember the volume dial.

Eysenck’s precise account has since been revised by others, among them Jeffrey Gray, and the notion that introverts simply have “more excited brains” has proved too crude. But the core idea, that people differ in how much stimulation they find comfortable, remains one of the most productive ideas in personality psychology.³ The volume dial is a helpful image, then. It is not a part you could point to on a scan.

The lemon test: charming, but not proof

There’s a small, classic experiment that is often wheeled out to illustrate this. Researchers dripped lemon juice onto the tongues of participants and measured how much saliva they produced.⁴

The result: introverts produced more saliva than extroverts in response to exactly the same amount of lemon juice. The best-known follow-up study found a correlation of around 0.7, which by the standards of psychology is a startlingly strong figure.⁵

And here I have to stop the story before it becomes too lovely. That startling 1967 study was by Hans Eysenck and his wife Sybil. In February 2020, the publisher SAGE and the editors of Perceptual and Motor Skills issued a formal expression of concern about it, along with dozens of Eysenck’s other papers in that journal. The trigger was a review by King’s College London of other work of his, on personality and illness, which had found serious problems with the data. That report did not name the lemon study, but the editors judged these papers problematic too, given how serious the findings were.⁶ On top of that, two researchers showed in 1970 that participants can deliberately control their own rate of salivation once they know what the test is about.⁷

So the lemon test is a charming piece of scientific history. It is not proof. I have left it in because you will come across it everywhere, and because you now know why it deserves a pinch of salt.

Dopamine: why a party rewards one brain more than another

There is a second difference, and it has to do with dopamine, a substance in the brain bound up with reward and motivation. It gives that pleasant “yes, more of this” feeling when something good happens.

The American researchers Richard Depue and Paul Collins gathered the evidence together in 1999 and described extroversion as a difference in the reward system.⁸ An exciting social encounter, a new experience, a gamble that pays off: the extrovert’s brain hands out a hearty reward for these things. No wonder extroverts go looking for them; their brain rewards them richly.

In introverts, that same system reacts more calmly. They draw their satisfaction less from external excitement and more from quieter sources. There is a popular theory that says another substance, acetylcholine, plays a larger role in introverts, fitting with inward-turned attention, calm reflection and focus. That theory comes mainly from popular books and is not firmly grounded in evidence. What you will recognise from everyday life: one person has a deeply contented afternoon with a book, while another grows restless after an hour.

Neither is better. They are simply two different ways in which a brain draws satisfaction from the world.

The big question: are introverts cleverer?

Here I have to be honest, even though I am an introvert myself and would rather like to say “yes”.

The answer is no. A meta-analysis that laid all the available studies side by side found a relationship between extroversion and intelligence that hovers around zero. In studies carried out from the year 2000 onwards it was even slightly negative, about −0.04, which in practice means: no relationship at all.⁹ Intelligence is more or less evenly spread across both groups. Anyone who claims otherwise is selling a comforting story, not science.

But here it becomes interesting: introverts often seem cleverer. And that is an entirely different thing. It isn’t about how clever they are, but about how they come across. Why is that?

They often think before they speak. Introverts process information more inwardly before they say anything. So what they do say tends to sound considered, not because it’s cleverer, but because it has had longer to ripen in the head before it came out.

They observe more than they talk. Someone who watches and listens a great deal notices things others miss. That looks like wisdom. Often it is simply attention: the introvert was quiet enough to see what was really going on.

They prefer depth to breadth. Introverts often favour one deep conversation over ten shallow ones. That makes their conversations seem weightier. Not because they know more, but because they would rather go deep.

Is there anything to see in the brain itself? Cautiously, yes, but differently from what’s often claimed. You sometimes hear that introverts have “thicker frontal lobes”. That isn’t well founded. A meta-analysis of fourteen brain-scan studies found that the individual studies contradicted one another sharply, and the regions that did emerge stably were linked to extroversion, not to a thicker frontal lobe in introverts.¹⁰

What has been measured cleanly: using PET scans, introversion was associated with more blood flow to the frontal “thinking regions” and to the front part of the thalamus, a relay station deep in the brain. Extroversion was associated with the front part of the cingulate gyrus, the temporal lobes and the back part of the thalamus.¹¹ So the introvert’s brain isn’t “bigger”. In this measurement it was simply active in different places, which fits with the tendency to think things over before acting. One honest caveat: that study measured eighteen healthy people. Eighteen is a small group to carry a large conclusion.

All that thinking does have a shadow side: it can tip over into brooding. And here a nuance often goes wrong, in both directions. In a large meta-analysis of 175 studies, neuroticism, the tendency to become tense, worried or low quickly, was the strongest correlate of anxiety and mood disorders. But low extroversion is also firmly bound up with them, most clearly with persistent low mood and with social anxiety.¹² An introvert is therefore not automatically a worrier. But introversion is not entirely off the hook either. Someone who thinks deeply inward and scores high on neuroticism probably runs the greatest risk of falling into that trap. No single trait is purely a gift.

Is the world run by extroverts?

This is a question that occupies a lot of people, and the answer is: partly true, but more nuanced than it looks.

The American writer Susan Cain wrote a famous book about this, Quiet (2012), in which she describes what she calls the “extrovert ideal”.¹³ In many Western cultures, especially in schools and businesses, extroverted behaviour is rewarded: the person who is loud, talks quickly, presents themselves well and fills the room often gets more attention, more opportunity, more recognition. The quiet, thoughtful person is sometimes overlooked, even when they have valuable things to say.

In that sense it holds true: our culture is built, in large part, on a preference for extroversion. Meetings reward whoever speaks loudest, not always whoever has the best idea. Classrooms reward whoever puts their hand up, not always whoever thinks the deepest.

But reality is richer than “extroverts rule”. Loud and influential are not the same thing. In two studies, one in a chain of pizza stores and one in the laboratory, teams actually performed better under a less extroverted leader, but only when the employees themselves were proactive. With passive teams the effect reversed exactly.¹⁴ The researchers’ explanation: a strongly extroverted leader is less receptive to other people’s initiative.

So no, the world isn’t simply run by extroverts. It’s more that our culture rewards extroversion more visibly, but visibility and real influence are not the same thing. Much of what shapes the world happens in quiet: in laboratories, at writing desks, in minds that would rather think than talk.

No one is a hundred per cent one or the other

One last important point: introvert and extrovert are not two separate boxes. It is a scale, a sliding line with two extremes. Most people sit somewhere in the middle. They are sometimes called “ambiverts”. They can enjoy a party and a quiet evening, depending on the day, their mood, and their energy.

Even the most pronounced introvert has moments of extroversion, and the other way round. It isn’t about a label that pins you down forever. It’s about a leaning: the side you naturally tilt towards.

What this means for you

The loveliest thing this science gives you is neither an excuse nor a sense of superiority, but something practical: self-knowledge, so that you can work with your nature instead of against it.

If you notice you’re an introvert: plan in some quiet after social commitments, not because you’re weak, but because your volume dial sits high and needs time to come down. Protect your quiet hours. Choose work and relationships that allow depth over bustle. And forgive yourself for being worn out after a busy party; that isn’t unwillingness, that’s your brain.

If you notice you’re an extrovert: make sure you get enough stimulation and contact, because too much silence actually drains you. But practise tolerating quiet too, because your brain also needs stillness now and then, even if it feels uncomfortable.

And if, like most people, you sit somewhere in the middle: learn to recognise what you need, and when. Sometimes bustle, sometimes quiet. Listening for which of the two today is asking for is a skill in its own right.

Not a flaw, not a superpower

I am an introvert. An observer. Someone who would rather watch and listen. For a long time I thought this was something to overcome, that I had to become louder, more sociable, more present, in order to count in a world that rewards the loud.

Science taught me something else. My quiet nature is not a flaw I must repair. Nor is it a sign that I am cleverer or deeper than others. That would be a vain lie. It is simply a different way in which a brain is tuned. A volume dial set high. A brain that loves depth and quiet.

And in a world brimming with noise, perhaps there is a real need for people quiet enough to see what is really happening. Not because they are better. But because every world needs both the loud voices that set things in motion, and the quiet observers who notice what the loud ones miss.

Both. Not one above the other. Both.

Are you more of an introvert, an extrovert, or something in between? And, more importantly, are you living in a way that fits how your brain is tuned, or have you been fighting your own nature for years?

No quick answer. Just a question to carry with you.

This article provides general information and is not medical or psychological advice. If you have questions about your own health or wellbeing, please speak with your GP or a treating specialist.

Related reading

• Why one person laughs and another throws a plate
• Why Rest Has Become Unbearable
• Your Brain Is Not an Honest Witness

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Sources

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11. Johnson, D. L., Wiebe, J. S., Gold, S. M., Andreasen, N. C., Hichwa, R. D., Watkins, G. L., & Boles Ponto, L. L. (1999). Cerebral blood flow and personality: A positron emission tomography study. American Journal of Psychiatry, 156(2), 252–257. https://doi.org/10.1176/ajp.156.2.252
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Sources on this site are cited in APA style, with a DOI wherever one exists. Why I made that choice →