My first assertion was that introversion of the first type can only exist in a situation where insulation is a possibility. Specifically, a situation of financial security. (Other situations which allow for such insulation exist, but these situations are uncommon and outside the scope of my argument.) Where contact with the outer world is necessitated, the introvert will, of necessity, develop social skills. Social skills are just that - skills. Just as a child can detest the piano but grow competent if his mother forces him to play every day, if you are forced to socialize to some degree every day, you will develop social ability to some end.
My final assertion was that, unfortunate though it may be, a member of an ethnic minority is more likely to be in a financial situation which necessitates socialization, at the workplace if nowhere else. Hence the notion of "white introversion" as opposed to "minority introversion", which I believe are analogous to the two classes of introversion I posited at the beginning of my argument.
I think you have a legitimate point here. My own experience is that "insulation" of the sort you mention does make people of all personality types rough around the edges and poorer at basic social skills than their counterparts who have greater contact with the outer world.
I deal with work-at-home contractors and sometimes bring them into the office for a month or more for training. Even the work-at-home extraverts can be a handful. They all get out of practice at dealing with people (or simply have never been in an organizational setting and learned typical office etiquette), and I have to rein them in and teach them the basics. The introverts in particular can be kind of sour or bitter or have paranoid concepts of how the office works. They just don't have any previous experience for comparison, and they attach significance to minor things in one area while missing out on big, glaring signals in other areas. And they don't necessarily adapt quickly or well. They often see themselves as "temps" or outsiders and merely try to tolerate things or simply keep a low profile until they can get back to their normal environment again.
By comparison, working environments (and even social circles) tend to "homogenize" the regular, full-time workers in terms of behavior, values, and ways of reacting to things. People want to belong, and peer pressure can be a significant force, so work and social circles homogenize their members to some extent over time. That effect is even more pronounced in blue collar work, which can be very communal; in a communal setting, peer pressure can be fierce and can push everyone toward one central "average." This homogenization has some bad aspects, such as pushing everyone into a groupthink model and punishing those who show too much independence (at least when interacting with the group). But it has good aspects in that it pushes people to socialize and stretch beyond their type. That's usually considered a form of "maturity" insofar as it involves growth, flexing, better overall social skills, and better adaptation to the realities of their social environment.
When I think back to my days in the military, I have trouble retrospectively typing a lot of people because they were trying on new "masks" and sometimes getting pretty good at them. The momma's boys were trying out tough guy roles, shy people were adapting to leadership roles, etc. Same thing in the office workplace today. Sometimes it takes me a while to type someone. Sometimes I have to mentally peel back a layer or two of learned behavior to see what comes natural to them.
By comparison the work-at-home types are often "clearer" (less socialized, less mature) versions of their personality type. They are more individualistic in thought perhaps, but it often seems to be an immature, egocentric type of individualism, and their world is often more circumscribed in terms of both exposure to new experiences and willingness to move out of their comfort zone in terms of action.
And I think this does seep into one's self-picture. Like any good introvert, I still like and need my downtime. But my self-picture includes the idea that I'm knowledgeable about office conventions and etiquette and can be comfortable in social settings, and that self-picture guides my actions accordingly. The work-at-home types often clearly don't have a broad self-picture. The introverts in particular get a deer-in-the-headlights look about them when asked to stretch into a new social or work environment.
Can this concept be applied to cultural groups as well? Probably--specifically where the groups are tight-knit, communal, and tend toward homogenization (via peer pressure) of the members. In those situations one will see more socialization, more flexing and adaptation to a social average, and better overall social skills. On the downside, one will also see more groupthink and less independence of word and action (at least when interacting with the group). Hence the group, when examined as a whole, may seem to tend toward one central homogenous personality type.
My first wife was second-generation American and had a very close-knit, working-class, old-world Italian upbringing. When visiting her family at home, it was easy enough to see that individuals had their own various personality traits; but the traits were often muddled and played second fiddle to the xSFJ big-family working-class Roman Catholic Italian stereotype. If tested with the MBTI, they likely would have scored very similarly if they all held roughly the same values in common. There was a lot of pressure to fit into well-defined, predetermined roles depending on one's position in the family. In the case of my wife, it took a bit of separation from the community and self-discovery for her natural ISTJ nature to come to the forefront.
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There are a lot of generalizations here, of course. But I'm describing what I've personally seen within the context of broad trends. Naturally, the appropriate disclaimers and qualifiers apply. [Edit:] And I should clarify that I'm talking about things in relative terms here. I'm not saying that at-home workers are objectively more neurotic than office workers. Both groups can presumably be perfectly healthy/happy/fulfilled/functional in their chosen environment. I'm just saying that full-time office workers tend to have better overall social skills, in my experience.