Very thought provoking answers, thank you. But do we have any insight into the chemical mechanism?
Maybe I am a bit socially awkward but it's irrelevant. I am comfortable. My friends accept me. No anxiety over it. But eventually if I can't get time alone, I'll start having small "blackouts" of a second ir two and become unable to retain information. It's creepy! I can calmly watch it happen. There is a real limit independent of my emotional state.
Marti laney in her "introvert advantage" book claimed innies rely on a slow, circuitous acetylcholine pathway while outies rely on a fast dopamine dominated path. Sounds overly simplistic, though, right? Ive not found any empirical support for this claim. She says acetylcholine is slow to recharge, hence the innie need to retreat. She says reflection builds back those acetylcholine stores.
This is getting to be an interesting thread. My first thoughts were running toward anxiety, like Logan said. Now, reading this, I'm thinking that there is really something else going on here. Laney's explanation does sound overly simplistic. Not to say it couldn't be at least part of the explanation, but people are always coming up with half-baked theories in the psycho-bio field and publishing them in non-peer reviewed fora.
Junk Master's hypothesis that this condition is the kind of thing that would be common in Aspie's seems very plausible to me. My life is full of people (both relatives and friends) on the spectrum; I even have a touch of it myself. (It's an occupational hazard in the science/tech world. Einstein would today be diagnosed as 'on the spectrum', but I digress.) I have a pretty fair sense of the spectrum and its various comorbidities, at any rate.
The drained feeling comes for the lack of innate social skills so the brain has to "work" rather than just react. See Thinking, Fast, and Slow by Daniel Kahneman.
Good news is, with enough work even those who would qualify for an Asperger's diagnosis can practice social skill sets and learn to handle them intuitively (fast thinking) and to the degree we can do that-- note I include myself-- we won't be as drained.
And then there's this hypothesis:
''Eysenck proposed that extraversion was caused by variability in cortical arousal. He hypothesized that introverts are characterized by higher levels of activity than extraverts and so are chronically more cortically aroused than extraverts. The fact that extraverts require more external stimulation than introverts has been interpreted as evidence for this hypothesis. Other evidence of the "stimulation" hypothesis is that introverts salivate more than extraverts in response to a drop of lemon juice.[16]''
''Extraversion has been linked to higher sensitivity of the mesolimbic dopamine system to potentially rewarding stimuli.[17] This in part explains the high levels of positive affect found in extraverts, since they will more intensely feel the excitement of a potential reward.
The part about introverts being more cortically aroused might make what I'm about to say seem wrong, but the higher dopamine sensitivity of extraverts suggests that introverts just need a little more of everyone's favorite chemical- Dopamine. I wouldn't recommend that you break out the crack pipe, but some Ritalin, Adderal etc. might help out here.