Three traits contributing to introversion are anxiety, social inability, and unusual, stereotyped interests (endogenous personality). Like most trait clusters, “introverts” are characterized by a combination of these, although the extremes of each trait produce very different patterns of behavior.
Most introverts are characterized by anxiety. Usually, it is social anxiety. Most introverts enjoy social interaction but they need time alone in order to “relax”- i.e. to down-regulate their anxiety so that it doesn’t become overwhelming. Afterward, they can return to social interaction and enjoy it. Such a person does not respond well to continuous low levels of social pressure, which will build anxiety up to the point that they freak out, but they may respond well to intermittent, high levels of social pressure interspersed with solitary periods for decompression.
This is the popsci understanding of introversion and it does a good job of describing at least 80% of introverts, in my opinion. Extreme introverts of this sort have an anxiety disorder- this is why Robert Lindsay hypothesizes that Asperger’s Syndrome, and the extreme introversion often observed, is a type of anxiety disorder. It is certainly true that most people with AS tend to have a great deal of anxiety. This is also why people at every job I’ve ever had have asked me “So, are you going to come in here someday and shoot everybody?” (Maybe I should be offended or something, but actually it’s just confusing.)
Some introverts are simply characterized by their inability to get along with others. This means that the anxiety of social contact is greater than the anxiety of loneliness. Really, this just means that a person like this is an extravert by nature, but they are either socially incompetent or very odd or both. It is very likely that this will cause oscillations between trying to socialize, failing, retreating, and trying again. Eventually, this personality will also likely develop anger issues, an anxiety disorder, and probably depression. Mass shooters almost all fall into the extreme end of this category.
Another sort of introvert is characterized by endogenous personality. An endogenous personality is characterized by an unusual pattern of anxiety which can’t be explained by external stimuli. This doesn’t mean they aren’t sensitive to external stimuli (probably they are), but there are also surges of anxiety that can’t be explained by the environment.
Werner von Braun is a good example of this- there is no earthly reason he should have been so obsessed with going into space. He wasn’t being hounded to become an astronaut by his overbearing Asian mother. His strange ambition was motivated by the anxiety he would feel by failing to go into space. The resulting “inner motivation” is the desire to quell this inexplicable, internally generated anxiety. Most people with Asperger’s fall into this category.