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If humans are hardwired to be averse to "millions of immigrants pouring into your home country," why is that? Presumably you're referring to that aversion being chosen by natural selection. That suggests that we're less likely to have been descended from those who were more open to displacement by outside populations, and more likely to come from those who guarded their borders. Societies that don't keep barriers are less likely to pass on their genes or memes to the future.

I'm assuming that the main difference between then and now is that an increase in resources means we can share freely with more "outsiders." Presumably resources aren't infinite, so what's the upper bound? How would we know?