Why Are Some People Introverts?

As travellers go around the world and report natural objects and phenomena, so faithfully let another stay at home and report the
phenomena of his own life.

Henry Thoreau

When I was growing up, some phrases stuck to me like glue.

‘You need to be more outgoing’.

‘And come out of your shell’.

‘People don’t bite!’

‘Don’t be antisocial’.

‘What do you do all day at home?’

As I got older, I realised that there were others like me and that there was a name for us: introverts. Since you are reading this, I’m assuming you are one too.

Introverts are people who prefer solitude to the company of others. But that doesn’t mean all of us are clones of one another—some of us are more introverted, some less. Plus, there is more to psychology than the answer to the extroversion-introversion question.

Like what? 

You have other personality traits as well, beyond extroversion-introversion. You also have intelligence, memories, goals, skills. Some people even have mental disorders. 

When someone says that he is an introvert, he’s simply pointing out a pattern in his behaviour—he enjoys being on his own more than his friends do. What’s significant about this pattern is that it’s an enduring pattern. Your personality will remain fairly stable over the years. You may, therefore, become a bit more or a bit less introverted in the next 20 years, but you will still retain a taste for your own company. 

I know these things. What’s the big deal?

I know you know! You have lived a life of introversion—you know all about the ‘what’ of introversion. But we are here to talk about the ‘why’. 

That’s the question I had in mind when I picked up Susan Cain’s book Quiet. It starts by pointing out something that I had suspected all along but never quite put into words. We live under the Extrovert Ideal. 

Let’s take a quick detour to Cain’s insights before we finally tackle the why of introversion.

An Introvert in an Extrovert’s World

Extrovert Ideal—the omnipresent belief that the ideal self is gregarious, alpha, and comfortable in the spotlight.
Susan Cain

Every parent wants what’s best for her child, but what does success look like in society? If it’s a person gliding through the room with ease and confidence, equipped with small talk and polite laughter, then introverts will have a lifelong handicap. The only success story possible for introverted kids is the one they can never achieve: to become extroverted adults.

Schools have started arranging desks in pods, further encouraging collaborative learning over quiet contemplation, says Cain. To get into business school, shining in a group discussion is an essential hurdle for candidates to cross. Workplaces are adopting open-office plans, driven by the belief that the best ideas come only from collective brainstorming sessions. 

Moreover, adopting extroverted qualities is frequently equated with self-improvement. The only desirable characteristics today are charisma and talkativeness, while solitude and reflection are best avoided.  

Cain’s point is not that there is something wrong with being extroverted, only that we should be free to express our true personalities. Pretending to be an extrovert is not the road to a fulfilling life. 

We are, however, on the march to becoming a society of extroverts and pseudo-extroverts. In the end, our world will become less diverse and lose out on the unique gifts of the introverted mind.  

Why Are Some People Introverts?

Knowing yourself is the beginning of all wisdom.

Our first step in understanding the roots of introversion is to learn a bit of personality psychology. Specifically, we need clear definitions for ‘facet’, ‘trait’, and ‘personality’. 

We have been talking about patterns of behaviour (like preferring to be on your own). Every such pattern is called a facet

Being an extrovert means you display a whole family of facets. Through large-scale studies, the psychologists Paul Costa Jr and Robert McCrae have identified some of these facets: warmth, gregariousness, assertiveness, activity, excitement-seeking, and positive emotions. What these facets essentially mean is that extroverts make friends easily, enjoy being around lots of people, like to be in charge, are energetic, are comfortable taking risks and are easily bored, and frequently experience happy emotions. 

In the book Personality, the behavioural scientist Daniel Nettle mentions additional extroversion facets: extroverts have many sexual partners; enjoy travelling; and are ambitious and competitive. (Heads up! I’ll be referring to Nettle’s spectacular book a lot.)

You would have noticed that several extroversion facets, surprisingly, have nothing to do with your social life—like wanting to travel. 

So what links all these social and non-social facets together?

Research has found that the people who are more social tend to enjoy travelling, competing, and sexual variety more than others. In technical terms, all these social and non-social facets are statistically correlated. So if I knew someone who loved parties, I would guess that he is also into travelling—but it may turn out that he isn’t. My point is that it would have been a good guess, because I would have had probability on my side (although not certainty.)

To identify these correlations, researchers create questionnaires that ask how often you engage in various behaviours (facets). These questionnaires are then given to lots of people and, finally, the researchers look for correlations in the answers they have collected. 

They always end up finding exactly five clusters of correlated facets. Each of these clusters is a trait. The particular trait that we are discussing is, of course, the ‘extroversion-introversion continuum’. 

How is it a continuum?

It is one because everybody has this trait, just to a different degree. Thus the question isn’t ‘If you enjoy making new friends.’ The question, instead, is ‘How much do you enjoy making new friends?’  

By measuring how-much, we can place every individual somewhere along the continuum between extroversion and introversion. The ones who fall closer to the introversion end are, as you might expect, the ones we call ‘introverts’. 

You said there were five traits—what about the other four?

Agreeableness-disagreeableness, conscientiousness-carelessness, neuroticism-emotional stability, and openness to experience-closedness to experience are the other four.   

Together, the five traits are called the ‘Big 5’. Personality is simply what you get when you consider where an individual falls on the continuum for each of the Big 5. Put plainly, the five traits combine to form your personality.

Being an introvert doesn’t mean that you are shy. Costa & McCrae found that self-consciousness was a facet of a separate Big 5 trait: neuroticism-emotional stability. 

Cain illustrates: 

In other words, you can be a shy extrovert, like Barbra Streisand, who has a larger-than-life personality and paralyzing stage fright; or a non-shy introvert, like Bill Gates, who by all accounts keeps to himself but is unfazed by the opinions of others.

The dictionary, however, continues to incorrectly define introversion by ‘shyness’.

Similarly, being an introvert doesn’t mean that you have poor relationships. Costa & McCrae found that trust, straightforwardness, altruism, compliance, modesty, and tender-mindedness were facets of a separate Big 5 trait: agreeableness-disagreeableness.

Nettle illustrates:

Extraversion is a predictor of how much a person will like going to parties, how much time will be spent in social activities, and the facility in striking up new friendships, but it is not a predictor of how well those friendships will go.

Your Brain

Everything we do, every thought we’ve ever had, is produced by the human brain.
Neil deGrasse Tyson

Our second step in understanding the roots of introversion is to learn a bit of neuroscience. 

All your thoughts and behaviours come from your brain. Thus there must be something about the structure of your brain that makes you an introvert. In a sense, saying that you are an introvert is akin to saying that you have an ‘introverted’ brain. 

What determines the structure of the brain?

The structure of the brain is determined by your nature (genes) and your nurture (childhood experiences). 

In The Blank Slate, the cognitive psychologist Steven Pinker tells us that the heritability of the Big 5 is 50%. What this means is that, both, your genes and your childhood experiences have played an equally important role in creating your introverted brain. (Researches have worked this out by studying identical twins.)

Which specific genes and childhood experiences create an introverted brain?

Figuring out how genes affect your behaviour is at the cutting-edge of molecular biology. That may sound exciting until you realise what it really means: scientists know very little about it. 

Nevertheless, there is a gene called DRD4 that helps the brain’s chemical messenger dopamine function. This gene happens to have variations, so the version of DRD4 that you have may be different from the one I have. 

Although we don’t know the details, Nettle notes that ‘The consensus at the moment is that there is some kind of relationship between variation in DRD4 and traits related to Extraversion’.

If we know little about the genetic influences on introversion, we know absolutely nothing about the experiential ones. 

Which experiences in early childhood lead to introversion? Or is it our very earliest experiences—the ones we had as foetuses in the womb—that make the difference? No one has the answers to these questions yet. 

We have finally found the first why of introversion:

Introversion is in part caused by the version of the DRD4 gene you happen to have (although there are probably other genes involved as well). Introversion is also caused by your early experiences (although the question of which experiences remains a mystery)

So far we have looked at how nature and nurture design your brain, but now we will look at the actual structures in your brain that are linked to introversion.

Nettle believes that the reward system in the brain is linked to introversion—specifically, a less reactive reward system may be the cause. The reason for his suspicion is a particular facet of this trait: positive emotions

Extroverts experience more happiness in all its forms: joy, excitement, optimism, anticipation, pride, engagement. And it’s the reward system that activates these happy emotions to make you work towards, well, rewards—rewards which come at the expense of hard work and risks. 

What are these ‘rewards’?

Finding a mate, having sex, being part of a group, and your status within the group are some of the big ones from an evolutionary perspective. In today’s world, money is also an important reward. 

Imagine you notice an attractive member of the opposite sex enter the bar and you get excited. As she laughs at your jokes, you feel the anticipation building. Finally, you get her number and are ecstatic about your eventful night out. 

The reactivity of your reward system determines how strongly you would feel the excitement, anticipation, and ecstasy in this example. 

If extraverts have a highly-reactive reward system, it would explain two things. First, it would explain why they experience more positive emotions in day to day living. Second, it would explain why they are more interested in seeking out new friends, sex, travel, and competition: they are promised intense emotional pay-offs by their brains. They engage in these facets more than introverts do because they get more of a kick out these facets than introverts do. To put it in yet another way, extroverts seek out objective rewards in the real world to obtain rich subjective rewards inside their heads. 

If introverts have a less-reactive reward system, it would explain why introverts seem aloof. The absence of positive emotions would make them emotionally flat and indifferent to the energetic adventures that society offers. 

In this way, Nettle points out, the reactivity of the reward system may be the reason why all those social and non-social facets of extroversion-introversion are correlated. 

Satisfyingly, the reward system uses dopamine, which means the first why we had discussed—about DRD4—is linked to what we are talking about now.  

Just because introverts feel less positive emotions doesn’t mean that they experience more negative emotions. Costa & McCrae found anxiety, anger, and depression to be facets of a separate Big 5 trait: neuroticism-emotional stability.

‘The opposite of joy and excitement is not fear and sadness’, Nettle explains. ‘The opposite of joy and excitement is simply the absence of joy and excitement—emotional flatness, if you will.’

The psychologist Hans Eysenck, on the other hand, has implicated an altogether different part of the brain. Cain introduced me to Eysenck’s hypothesis about extroversion-introversion: introverts have a high-bandwidth ascending reticular activating system (ARAS). 

An introvert with her high-bandwidth ARAS receives too much information about her surroundings. Hence, she seeks out low-key environments—spending weekends at home or going out to dinner with close friends, for example. The high-stimulation situations—such as talking to a large group of strangers—can quickly overwhelm her. In short, the introvert will look for ways to save herself from the discomfort of over-arousal.

An extrovert is the opposite; she has a low-bandwidth ARAS. Reading quietly in her room or gardening is boring, but she will feel engaged at a loud party or a concert. (Cain lists out the elements that make a situation highly stimulating: ‘It can take any number of forms, from noise to social life to flashing lights’.) In short, the extrovert will look for ways to save herself from the boredom of under-arousal. 

If Eysenck’s hypothesis is true, it would mean introverts are comfortable doing one thing at a time and are good at concentrating, making them suited to intellectual pursuits. Extroverts, on the other hand, would be good at multitasking.

We have now found the second why of introversion:

Introversion may be caused by a less-reactive reward system or a high-bandwidth ARAS. 

I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection.
Charles Darwin

To travel deeper into the heart of introversion, we need to talk a bit about evolutionary biology. 

The naturalists Charles Darwin and Alfred Wallace were the first to explain how characteristics of organisms change over generations.

Parents pass on their genes to their children during reproduction. There are sometimes errors in this process—errors in the copying of genes from parents to their child. And that’s how you get different versions of a particular gene; it’s a copying error

Genes, of course, affect the characteristics of the organism and, most of the time, these new versions negatively affect the ability of the child to survive and reproduce. Every once in a long while, however, a new version appears that happens to work better than the original—thereby increasing the reproductive potential of the child. 

This new version may, for example, help the child digest food more efficiently, so he would have to spend less time finding and eating food than his peers. This would leave him with more time for the other activities necessary for reproduction, such as finding a good mate and investing in child-rearing. 

Hence, such a fortuitous child will have a greater-than-average number of children, in his turn. And so will all his children who have this improved version of the gene. In this way, over thousands of generations, the improved version will gradually become more and more common in the population.

In one famous sentence, Darwin sums up this process and even gives it a name: ‘I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection’.

The corollary to Darwin’s statement is that unhelpful variations will, slowly but surely, be weeded out of the population. 

How is this relevant to introversion? 

Nettle believes that natural selection can explain the extroversion-introversion continuum. 

We know that where you fall on this continuum is, at least partly, a question of genetics. And if there were a place on this continuum that was the best, then natural selection would have ensured that we all had genes that steered us towards that optimal point. 

If extroversion were superior to introversion, then why do we still have versions of genes that push us towards introversion? Similarly, if introversion were superior, why do we have versions of genes around that push us towards extroversion?

The answer that Nettle proposes is that neither extroversion nor introversion is inherently better. Perhaps, in some environments, it pays to be an extrovert, while in others, it pays to be the opposite—an introvert. Such a phenomenon is a special case of natural selection, known as fluctuating selection

Can you give an example?

This dynamic does indeed play out among our animal relatives. Nettle has described the research of Niels Dingemanse and his team on the bird known as the great tit

In the years when food is plentiful, extroverted great tits fare worse—they take unnecessary risks because of their desire for exploration. (Nettle goes so far as to nickname extroverts ‘wanderers’.) Yet in the years when food is scarce, the introverted great tits fare worse—they are less likely to leave their neighbourhood in search of food.

Add to this the insights from Nettle’s own research—this time on humans. Extroverts, on average, have more sexual partners and more hospital visits.

Putting everything together, we see that extroverts follow a high-risk-high-return strategy. In some environments, the returns are worth the risk, while in others, the risks are simply unnecessary. 

It would also mean that, in the long story of human survival which spans millions of years, both our extroverted and introverted ancestors have played complementary, yet starring, roles.

This is just a hypothesis, of course. What is interesting is that Nettle is the only scientist, as far as I know, to have tried to answer the evolutionary-why, indeed the ultimate why, for the extroversion-introversion continuum.

Introversion may be caused by the phenomenon of fluctuating selection.

I can’t help but feel that the world today is an extrovert’s paradise. And I wonder if, over the decades, the average extroversion scores for the global population have been increasing. If they have, it might explain the Extrovert Ideal. Perhaps, the conditions now do really favour the wandering extroverts among us, and their genes are on the rise. 

But where does that leave us, the introverts? 

Remember fluctuating selection— the environment has kept changing throughout our history, and the tables have kept turning. Who knows what tomorrow will bring? It might be a world where the dynamics of survival are very different from those of the last century. (Maybe the best advice for introverts is simply to hang tight?) 

It was my cousin who introduced me to the next why. We were talking about introversion when she pointed out that people behave differently in different situations. 

Despite my introversion, I talk a lot with my mother—because she is someone whom I love and trust. And despite my cousin’s extroversion, she isn’t talkative at all if it happens to be a busy day. In this way, my cousin reasoned, it’s the situation that determines behaviour, not personality. 

This argument, called situationism, was first put forth by the social psychologist Walter Mischel. It has an undeniable element of truth. The situation is indeed a better way to guess how someone would react in that particular situation. 

Here is the situationistic-why, which I will argue against:

Introversion is simply a reaction to certain situations. And as the situation that you are in constantly keeps changing, there is no such thing as stable introversion. 

Yet if you were to watch my cousin and me for a week, you would see that she is frequently talkative, whereas I am frequently quiet. In other words, ‘talkative’ and ‘quiet’ are patterns in our behaviours.

Certainly, patterns are weak predictors of behaviour. They can’t tell us what you would do in a particular situation. However, they can tell us what you would do in most situations.

Furthermore, in the paper Selection, Evocation and Manipulation, the evolutionary psychologist David Buss explains how your personality can change the situations that you find yourself in. 

Normally, I decline invitations to parties, while my cousin accepts. This is an example of ‘situation selection’.

Even if we were to both accept, our varying personalities would still colour the situation differently. Smiling comes naturally to my cousin and, hence, she is greeted by people smiling back at her. As I typically look pretty serious, I find neutral faces looking back at me. This is an example of ‘situation evocation’.   

Putting it all together: 

Your personality and the situation you are in, both, are useful in predicting your behaviour. Moreover, your personality affects the situations you end up in. 

Therefore, the situationistic-why is invalid and doesn’t tell us anything about the cause of introversion. 

At last I will devote myself sincerely and without reservation to the general demolition of my opinions.
René Descartes

The final why comes from philosophy. 

There is a belief that is so common that the philosopher Gilbert Ryle calls it the ‘official theory’. The belief is simple: every person is essentially a soul inhabiting a body. 

And the soul isn’t physical in nature—it isn’t made up of atoms and molecules, like everything else. It’s something completely different, something non-physical in nature. As all of science is the study of physical stuff, the soul is, and will remain, unknowable

‘It is invisible, inaudible and it has no size or weight’, observes Ryle. ‘It cannot be taken to bits and the laws it obeys are not those known to ordinary engineers. Nothing is known of how it governs the bodily engine.’

Famously advocated by the 17th-century philosopher René Descartes, this worldview is called dualism. 

Dualism assumes that your soul has complete authority over your behaviour. Situations and your brain don’t constrain your soul in any significant way. In short, your soul can do as it pleases.  

This leads us to the dualistic-why, which I will argue against: 

Introversion is caused by your soul having chosen introverted behaviours. And it could just as easily have chosen extroverted ones. 

In opposition to dualism is the worldview of materialism, which tells us that everything that exists is physical in nature. There is only one kind of stuff. Materialism implies that your consciousness is nothing but an effect of your brain and, regrettably, the soul doesn’t exist.

Furthermore, materialism suggests that all your behaviours are the result of mechanical information processing. If I had complete knowledge of your situation and your brain, I’d know exactly how you were going to react

Intriguingly, there is no way to figure out which worldview is correct. But here are three problems with the dualistic worldview. 

In Consciousness Explained, the philosopher Daniel Dennet explains the first problem, which he calls the ‘standard objection to dualism’. Let’s consider how any behaviour would take place under a dualistic model. 

Your five senses capture information about the situation and convert it into electrical signals. Thanks to your nerves, these signals travel at lightning speed to your brain. Your brain then analyses these incoming signals to understand the situation. So far, so good!

Now, your brain has to somehow pass on its analysis to your soul. But how? We don’t know, since we know nothing about the laws of the spiritual world. (Perhaps, there is some mysterious way for physical things to communicate with non-physical things.) Moving along to the next step!

Your soul receives your brain’s report and decides how it wants your body to react. In the next step, your soul must send its commands back to your brain for immediate implementation. This step is the first problem.

While we don’t know anything about the spiritual world, we do know the physical one. The laws of physics tell us that you can’t create new energy. How, then, does your non-physical soul—which wouldn’t have any physical energy—communicate with your physical brain? To successfully convey its commands, your soul must cause a change in your brain, i.e., create a small electrical charge. And this change would require the creation of new energy.

Dennet sums up the first problem with dualism: ‘This confrontation between quite standard physics and dualism has been endlessly discussed since Descartes’s own day and is widely regarded as the inescapable and fatal flaw of dualism’.

Thankfully, the second problem with dualism is simple. If scientists could explain your consciousness and behaviours by studying your brain, then there would be no need to assume that there is a soul at all. Although scientists are not quite there yet, they are making good progress. 

The second problem with dualism, in Pinker’s words:

But it is still tempting to think of the brain as . . . a control panel with gauges and levers operated by a user—the self, the soul, the ghost, the person, the ‘me’. But cognitive neuroscience is showing that the self, too, is just another network of brain systems.

The final problem is the simplest—dualism is an intellectual dead-end. By its mysterious non-physical nature, the soul will remain forever beyond the realm of scientific enquiry. 

The third problem with dualism, once again, in Dennet’s words: ‘[T]he few dualists to avow their views openly have all candidly and comfortably announced that they have no theory whatever of how the mind works—something, they insist, that is quite beyond human ken.’ 

No one can disprove dualism. Because of its three problems, however, I choose to ignore dualism. Along with it, I’ll ignore the dualistic-why. 


We’ve covered all the whys. Hooray! 

You now know, in detail, what introversion means. Plus, you know what causes and what doesn’t cause introversion. You may even have learnt some things about yourself. 

Thanks for reading till the end. 

Introverts of the world, unite!

Here are two additional articles of mine that cover topics related to introversion: ‘What Is Solitude?’ and ‘Understanding Loneliness‘.