When do you take on the nationality of a country?

In America this seems a particularly pointed question because there is such a wondrous hodgepodge of ethnicities here. Take my niece, for example. She was born, by chance, in California but returned to Africa as an infant, and she grew up in Cape Town. She didn't put foot in America again until she was in her 20's. By birth she is American, no question. She has an American passport, is married to an American, is raising three American children, and pursuing post-graduate American studies. To all intents and purposes she is American. And yet, not quite. She has a whole wealth of an African upbringing and, although she has acquired some American inflections in her speech, she has, essentially, a Cape Town accent.My case is even more of a question-mark. I was born in South Africa, had my early schooling in Namibia, went to university in Cape Town, and lived and worked there and in Johannesburg, before putting foot in America for the first time in 1995. I immigrated in 1997 and became a naturalized American citizen in 2001, so I am an American. And yet, not quite. I never for a moment lose sight of the privilege of being able to live in this country and I care passionately about what happens to it. But when I return to Africa, the first smell of it, the first sound of a South African accent, and I am immediately transported back to the familiarity of it all, even though I haven't lived there for thirteen years. The backlog of my history is African and there are huge holes in my knowledge of American culture. I couldn't watch the first man walk on the moon because television was still banned in South Africa in 1969, I never watched Johnny Carson live, I only obliquely understood the complexities of the Vietnam war. And I need only open my mouth for people to know that I am a foreigner, an outlander, even though I am, legally at least, an American citizen.And what about some of those famous emigres: the scientist, Albert Einstein (Germany), the tennis player, Martina Navratilova (Czechoslovakia), the choreographer, Georges Balanchine (Russia), the actor, Anthony Hopkins (British), the composer, Igor Stravinsky (Russia)? They are Americans. And yet, not quite. Don't we think of them first and foremost in terms of their countries of origin?So when, if ever, does one intrinsically assume the nationality of an adopted country?