I think it human nature to want to identify with a particular group.
agree with that statement and as a proof of that we should notice that each person identify himself with:
- street where he lives (in £ódź some hools ask you where you live and if you are "swój" they leave you alone)
- district where he lives (the same as above)
- city/town/village where he lives (there is sometimes rivalry between cities; in Poland Warsaw vs rest of Poland)
- region where he lives (western poland vs eastern poland)
- country where he lives (poland vs other coutries)
- continents where he lives (america/usa/ vs europe)
So whatever you would done ppl will always try to make some groups and compete with others
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As for nationality determination:
its about personal declaration. If Seanus feel Japanese no one can forbit him to feel Japanese. There some kind of trust that these declaration of nationality won't be ridiculuos (and mostly are not). It's the legal view
For me personally I agree that I can't reject anybody's self-identity right but I feel that a person who wasn't brought up in Poland for some time is not fully polish. It's because being raised in different country makes you that you don't understand fully the every little nuance of "culture" or behaviour you can encounter in Poland. You can try to come from behind in these aspects as an adult but adult has fully shaped mentality and it can happen that he never will feel here like at home. I think that good example of that is NMP who seems to be quite dissapointed of Poland. He was raised in different country and even though his grandparents and parents were trying to brought up him in touch with Poland it seems they did not managed to show him real Poland. Now as an adult he sees only the negatives and almost everything in UK seems to be better for him. Of course I don't know NMP family or even him and maybe I'm wrong but that's the impression I have.
The good example of that is the movie "American rhapsody" (based on true story). Even though the main charachter was born and raised to an early age in home country (Hungary), she couldn't find her place there when she came back there from the USA as a young lady.