[vkk8]

Neither is being introverted/extroverted.

Our mind is more complex than simple dichotomies like this assume. We all behave like "introverts" in some situations and like "extroverts" in some situations. We constantly adapt to our environment and our responses to external stimuli are not stable in time. They also depend on our internal state like hunger, tiredness, etc. much more than these sort of classification schemes would have you believe.

[nicoburns]

> We constantly adapt to our environment and our responses to external stimuli are not stable in time

This is true, but it's also true that peoples' average preferred amounts of stimulation are very different to other people's. And those averages often are quite stable.

[mgh2]

The OP statement is very interesting, will need more resources to confirm. Related: https://news.ycombinator.com/item?id=30010119

As for introvert/extroversion: On Susan Cain's book "Quiet"

The "Galen’s Prophesy" reveals certain behavioral predispositions of children within four months old: https://www.youtube.com/watch?v=CVJBzvaylH8&ab_channel=JillB...