I have come to the realisation that the introvert label is a coping mechanism for those with low social capability. Capability in this context relates to how comfortable you are expressing your thoughts in social situations, how easy/difficult socialising feels and the length of time you can socialise for.
My belief is that introversion stems from social anxiety and the behaviours we associate to it are coping mechanisms to explain why we aren't as capable as our extroverted brethren. We prefer being alone because interacting and connecting with people and groups is difficult and challenging. We don't like to be the center of attention because the fear of failure or making a mistake is anxiety inducing. We feel tired and need time off to recharge because we use up too much working memory when socialising (conscious in/competence?).
For a long time I believed in these coping mechanisms, despite spending half a decade trying to become more extroverted. I've observed my extroverted friends who are fearless in social situations, who effortlessly command entire groups, say whatever is on their mind without hesitation, and seek encounters like the social animals that we are. And then it dawned on me. This is purely a matter of skill and experience. Nothing else.
They have far more positive social reference experiences, which in all likelihood goes back to their childhood where they (probably) grew up in an environment free of judgment and repercussions. They learned to say whatever was on their mind so interacting with people was an extension of that. They learned that communicating and interacting is normal and natural and that anxiety is not a normal response. Now I know what you're going to say, anxiety and introversion are not the same thing.
So riddle me this, I exhibit all of the feelings and behaviours associated with introversion. I took an anti-depressant one day and it completely transformed me. Words flew off my tongue without a single thought, I initiated social interactions automatically like my life depended on it, I felt energised despite spending an entire day communicating with people. My job is people facing and I spoke to over 15 different people that day. I had people circle around me and I felt completely at ease being the centre of attention. What had changed? I'll tell you what, for the first time my life I was without any anxiety.
Despite this comment coming off a little bit simplified, I think you are absolutely right. I have had pretty severe anxiety when I was younger, and I might have easily been termed 'very introverted' as I spent almost all my time on my own, had very few friends and had to 'recover' from all kinds of social interactions.
I have never liked the terms introverted and extroverted, I believe they are labels for symptoms. The same way that now there is a category for 'emotional intelligence' or fixations in psychology on particular categorizations of people. I don't believe such things exist as concrete phenomena, but may arise in any degree based on physiological differences or psychological habits. Physiological differences in my opinion are usually minor - we all know times when we have experienced all types of these personality categories, so it seems strange to me to assign categories to something as fluid as emotional state. This is why believe there are two things that account for the vast majority of a person's state of mind:
attention control and self expression.
What you got from the anti depressant is really just dopamine, you probably can get it from any intoxicant. But dopamine is not something that is supposed to be there at some 'normal' level, it comes and goes based on a person's internal reality and how they place themselves within it. And a person's internal reality is based on where they place their attention (is it on constructive/positive things or negative things?) as well as how inhibited they perceive expressing themselves to be - because the mind is integrated not by merely 'believing' but through action and words that conform to those beliefs - when a person watches themselves doing something other than what their internal truth dictates, their mind is not untouched or unchanged by the experience, it creates a lot of tension and anxiety, which is why some people can be troubled and even avoidant of success when their internal reality dictates they should not be in that position.
As with all psychological things, it is a chicken and egg problem - you can influence things both by changing dopamine artificially or by habituating your attention and emotional response (e.g. through meditation) as well as becoming 'in touch' with yourself - which is really just being in the habit of developing a strong identity and belief system and being in the habit of expressing it.
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The internal world of the mind is vastly more complex than people tend to think. The unconscious mind computes most of our behaviours and actions and emotional state without our intention or conscious input, and holds a reality which we cannot change except by attrition over long periods of time, like water forming a rock. And we can hold disfunctional conceptions of ourselves and the world around us totally unawares, that lurk in the unconscious as neurosis. Something like PTSD is only the most obvious incarnation of a disfunctional internal reality that is consciously out of reach, but I believe such things operate throughout the mind at all levels of severity, from inhibitions that we don't even realize are there, to something like an all out panic attack.
It's one of the greatest shames of the 20th century, in my opinion, that psychoanalysis faded out of existence, since it seems like one of the most irresponsible things to me to not develop a conceptual model (not a physiological one) of what happens under the surface of conscious thought, given that the conscious mind is merely a tiny fraction of what exists there outside of our direct control.
As Carl Jung eloquently put it in Man And His Symbols:
"The motto "Where there's a will, there's a way" is the superstition of modern man. Yet in order to sustain his creed, contemporary man pays the price in a remarkable lack of introspection. He is blind to the fact that, with all his rationality and efficiency, he is possessed by "powers" that are beyond his control. His gods and demons have not disappeared at all; they have merely got new names. They keep him on the run with restlessness, vague apprehensions, psychological complications, an insatiable need for pills, alcohol, tobacco, food - and, above all, a large array of neuroses"