I am very, very introverted. I only really open up with a few people, and if I don't know anyone very well in a given group, especially if I'm completely new, I will quietly take out a book and read until it is time to go or I have to start working or someone I know comes in. This can be a problem, especially when I need to make friends or contacts, because I get very stressed when I have to talk with people I don't know well--I'm terrified of saying the wrong thing, but I'm even more terrified of having to strike up a conversation. Online is easier. I can just hide for a while if I need to.
Interestingly enough, extremes on this seem to run on my family. My dad and younger sister are total extroverts--they love people and, especially my sister, absolutely hate being alone. My grandfathers and paternal great-grandfather, on the other hand, particularly my maternal grandfather, are/were very introverted, as much as I am. Recently my family threw a surprise birthday for my maternal grandfather--who promptly fled for the basement as soon as the festivities were over.
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cogito, ergo sum.
non cogitas, ergo non es.