Are you an extrovert or an introvert? If you've ever done a Myers-Briggs test, you are familiar with the questions: Do you prefer being in a crowd when you are stressed, or would you rather retreat? Do you have lots of friends, or just a few, very deep ones? Do you prefer a loud party of 100 or an intimate gathering of 4?
While in parts of the business world the Myers-Briggs Type Indicator is almost a religion, it was originally meant as a tool to explain that different people think differently, and that these differences of manner shouldn't be read as differences of attitude. It's not that a Perceiver is indecisive - it's that a P needs data to form an opinion. It's not that a J is impatient, it's that until the thing is finished, it may well not exist to her.
The I vs. E dimension, introversion vs. extroversion, is something many people easily identify with, much more so than the other three. In fact, while all other ones require explanation just for anyone to understand what they mean, with this dimension it's the other way around: people identify first, and then they usually need to be explained what the dimensions mean.
The problem with this dimension is that many people that aren't on the ends of the spectrum don't quite know what to make of the classification. They feel neither introverted nor extroverted, but they don't particularly identify with other "zeros". What's going on?
Primary human emotions are few and powerful. Research tells us there are six of them: happiness, sadness, fear, anger, surprise and disgust. They are very distinct and come from different places in our brain, having evolved out of reactions to external influencers. Fear, for instance, is an emotion meant to deal with threats. Disgust is supposed to prevent us from doing things (usually: eating them) that could be dangerous for us. Surprise is the way we deal with the unexpected and opens us up for new information.
None of these deals with introversion vs. extroversion. You could say, that's because that dimension does not belong to the six primary emotions and is a method of processing. Instead, I propose that's because introversion and extroversion are actually dominated by several of these emotions.
I noticed for a while that stereotypical extroverts react to the information that I am unattached with a type of horror. They don't know how I can deal with that, and aren't I afraid I am going to end up alone?
I just smile, but I note that the reaction is not one of displeasure, or driven by lack of happiness. It is one of fear. Those are two different primary emotions, and I note that these stereotypical extroverts express fear of loneliness. A fear I don't have and never had had.
Introverts, on the other hand, are painted as afraid of others. Their seclusion is explained as shyness, a form of fear of the unknown, expressed as fear of unknown people. This explains the questions about the number of friends: an introvert would be afraid of unknown people, but not of known entities.
But is this all about fear? Is that what distinguishes extroverts and introverts? Extroverts are such only because they are afraid of being alone, and introverts follow their lonely path because they are afraid of others?
It's not that simple. The questions asked also open up a happiness dimension. Extroverts, in those questions, enjoy the company of others and seek out people because it's more fun than being alone. Introverts, on the other hand, enjoy doing things on their own.
So you have two extremes: typical extroverts, who are afraid of loneliness and enjoy doing things in company, and introverts, who are afraid of people they don't know and enjoy doing things in isolation.
What about people that mix that all up? For instance, you could be afraid of loneliness but enjoy doing things in isolation, or you could be afraid of people but like doing things in company. What you like doing, what gives you happiness, is only weakly correlated with what you fear. In particular, the correlation is likely to be driven by the role of fear in your life: people who aren't affected by fears as much will tend to not correlate on the introversion / extroversion scale, while people that are driven by anxiety and fears will tend to align more strictly with these dimensions.
Let's assume you aren't afraid much. Let's also assume you are afraid of (unknown) others, but like being social. As mentioned, that's easily possible if the fear of others is not much of a factor for you. Your tendency would be to be with people in situations where you don't have to interact with them. You might like going to church, or watch movies in a theater, or sit in a cafe all day, watching people pass by.
On the other hand, you might be afraid of being alone, but like doing things by yourself. In that case, you would probably interact with others when needed (and maybe be pretty good at it with the studied competence of the person that has to learn why people behave the way they do). When you don't have to, though, you'd disappear and do your thing. I would expect that kind of person to hang out in places where they can easily disappear without being noticed and where they are in the crowd, but don't belong to it - teachers, maybe, or engineers.
I am the first kind: I love having people around me, but I learned that I generate mutual boredom with a lot of those I met. It's not that I dislike the average person, it's that it is quickly mutually agreed we won't work out. So a really good place for me is the gym: I can work out surrounded by people, but I don't really interact with any of them. That gives me odd satisfaction, and a day when the gym is empty makes me unhappy. Notice that the quality of the people doesn't really matter - it's not that I am looking for specific people. I just want to have people around me.
I call people like myself intrexes, or introverted extroverts. My kind likes company and is extroverted that way, but for whatever reason (usually history), we shy away from random others. Maybe we are socially awkward, maybe we are outliers in our culture (try not liking soccer in Italy, or not watching TV in America!).
The opposite of my kind are extrins, or extroverted introverts. That kind doesn't particularly enjoy the company of others, but is able to function successfully in a social environment, fooling everyone into thinking they are extroverts. You know the type: the class clown, the realtor, the consultant. People that flow with the go but disappear suddenly to do what they really like doing, namely accomplish things by themselves.
Intrexes and extrins are as far apart from each other as stereotypical introverts are from extroverts. Motivation and punishment are just as misaligned for the first pair as the second. That's why this zero area between introverts and extroverts is so unfocused: those who live there are a very varied bunch. In fact, an extrin has more in common with both an extrovert and an introvert than with an intrex, and vice versa.
So if you routinely zero out of a Myers-Briggs introversion/extroversion scale, you need to ask yourself, are you an extrin or an intrex? Because who you are changes your game completely.