People have a great tendency to expect to find underlying truths. Introverts and extroverts are *really* like this underneath. Men and women have blah blah bullshit different brains. Etc.
But my idea of introversion is almost the reverse. I speculate that it's best understood as a catch-all for people who are less social for whatever reason. Two axes I think of (I don't know if this makes sense for other people) is "how much you NEED interaction with other people" and "how EASY you find interaction with other people".
And some of that is who you are, and some of that is circumstance: lots of external factors can make socialising easier or harder, which forms a self-reinforcing feedback loop in how easy you find it. This would predict that some people who aren't that interested, some people who naturally find it difficult, and some people who are prevented by circumstance, are similar in many ways.
And it also ties into the "extroverts gain energy from interaction, introverts spend energy on it" idea which many people endorse. In my way of thinking, that's more of a consequence than a root cause, that you need it a certain amount, and it takes a certain amount of effort to do, and if it refreshes you more than it costs, it leaves you net positive on energy and if it's the other way round, you need a reserve of energy to spend on it.
For instance, I notice with Liv and I, when we're interacting with each other, we need quite similar amounts of time. We can spend a *lot* of time just interacting, but we both need a certain, not that large, amount of time having a break from it too. But it seems to me, Liv is like that with *more* people. Whereas the number of people I can interact with basically indefinitely is quite small.
So my theory is, some people don't *need* that much social interaction, whether or not they find it easy when they need to do it. And other people find it difficult to varying degrees, but act quite similarly when they're with people they *can* interact easily with, but vary in how often they are.
But I don't know if that sounds like it applies to other people, or just how it helped me to think of it.
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