You could have a hole nature vs nurture debate about it; I personally think tho too that you are basically born that way. If dealing with mbti topics you always have to remember that it is about preference. You could for example easily find out whether my girl and me are Ts or Fs by just observing our TV habits. If she has the remote we always watch shows about people, where people talk about problems or where people are on their jobs; she is absolutely focused on that and it is what intrests her. When I have the remote we watch documentaries, political discussions, news or science - fiction and she always falls asleep then, because it is not what intrests her. It's not that this is an all-or-nothing thing, my girlfriend for example talks politics with me aswell, but when she does she does it very briefly and most often only to please me. I do talk about people problems with her aswell, but when I do most often it's very briefly cause I get headaches after a time
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So mbti is about preference, it doesnt really reflect 1on1 what atoms you are made of, but it tells more about what intrests you, what you want and what your personality is like.
Regarding N and S difference I have an example too. We lately had a team discussion at work and I visited a week before an event in which the topic were energy supply concepts of the future. The company I am working has it primary intrest in our hometown, it looks for companies to settle down here and is occupied with keeping the economics at our location alive. So I told in the team about a guy from the oil company Shell I met on the event, which said that Shell will produce by the end of 2011 a new fuel called GTL. It's a bit better than normal fuel, has a higer energy density, does produce less CO2 and lets the engine run smoother. And it's made from natural gas instead of oil. Well after I reported that a guy from my team asked me what this has to do with our work; which is of course concentrated on our home town.
Well that guy of course was right, the direct object has nothing to do with our work, but he failed to see the relation to the impact it will have on world economy which does affect us. If there will be alternate fuel concepts, things like electrical mobility for example will be less important, which does directly affect our work cause we are working in a huge network of companies who try to realizte electrical mobility in our hometown.
An isfp that day rushed to my help and explained the guy from the team that. The isfp has been hanging out with me for so long already that his intuition has gotten stronger. He still isnt an intuitive guy cause his main focus is very down-to-earth and I am dependant on him very often so I dont overcomplicate things for myself too much.
Of course what you have here, the whole mbti thing, is kinda very concentrated on a persons life in society and your career for example. If you were to just talk about a friend you play for example computer games with, you'ld use completly different attributes than intuition to describe him. So the mbti model is one attempt to systematize peoples preference in a society and to create a conceinvable picture for exveryone what different personalities there are in the world and how people are motivated. But it's definitly not the only answer to speak about personalities and it is no psychology, it doesnt tell with symptoms it only reflects clearly that what you told it.