It's an issue because it has been going on since air transportation became a feasible way of gene flow.
I think that the former post and this quote, deals with Eugenics, and well, ideas of racial superiority.
I personally do not want to deal separatelly "cultural" and "racial" issues.
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In the beginning all was fine, you experienced new people, nothing was harmed. Over time, more and more of those people came into your towns. Now they try to over-run the neighborhood, they don't contribute, and they stay to themselves. They don't speak the same language. Now what? They commit more crime, the people have to pay their welfare, medical care, etc. They take more money from the economy than they produce.
This is a pretty well summarize the compound of fears, real or imagined, that people in receiving countries have, and effectively make Multiculturalism (I rather say, massive immigration) a so actual issue.
However, whole countries like Australia, Canada and the USA were constructed that way... with massive immigration.
About the USA... If they had only received the initial wave that constructed the 13 colonies of New England they could have not be able to effectively populate the center and the west (for good or wrong).
Economic times are hard, but getting better. People don't want to pay for people that have little genetic relation to them. The question of multiculturalism comes up.
O.K.
However, it is difficult to say if in all the cases immigrants improve or worsen the economic situation.
It could not be proven, for you cannot put or take out the people to test it. And even if you do, you cannot control everything, and say that a point about it, is completedly proved.
What it is truth, is that many people in the receiving countries feel these fears/grevances, and that generate hate... against people generally more weak. It helps the creation of radical nativistic political movements.
Thererefore, I believe that when it is posible, the people in those countries that plan to migrate, have conscience that the best option will be to stay and try to improve their own countries.
If there is not massive immigration, the reaction of rejection could be less fueled, and the good experiences of @Antigone, as multicultural emigrants, will be more generalized.
Regards.