Did this study consider the difference between white and non-white immigrants to mostly white Western countries?
Yes, the effect was very pronounced in Africans, possibly due to the use of the traditional African stimulant khat. But white people do drugs too, so this seems very dubious.
Did this study consider the difference between white and non-white immigrants to non-white countries?
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Did this study consider the difference between immigrants who (try to) assimilate to local communities, and those who prefer to stay within national communities?
Yes, the effect was very pronounced in Africans, possibly due to the use of the traditional African stimulant khat. But white people do drugs too, so this seems very dubious.
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No, although maybe other studies did.