I do wonder, could be that for much of human history if you ever saw a foreigner one of you was about to die. PNG blood feuds being a modern example. Perhaps societies with a strong centralized state would evolve a culture open to integrating foreigners. I also wonder how much of modern multiculturalism has its roots in colonial empires.

AFAIK many of the Scandinavians that came to the US were escaping the really bad ethnic / religious stratification and famines.