IQ studies suggest that at least part of the problem in big under-achieving populations like sub-Saharan Africans or Native Americans is that these populations don’t contain enough smart people. Now you want to lure away the few smart people they have?
And then there are those results from psychology telling us that unconscious biases cause great numbers of us to be uncomfortable around persons of obviously different deep ancestry. (Presumably a factor in Prof. Putnam’s findings on diversity. [16.5] By the way, I have posted my own results from that Harvard test here. It would make discussions of these topics a bit more interesting, and a lot more honest, if more people would do the same.) It may be that, even if you take job skills and IQ out of the equation, multiethnic societies can’t be stable. Perhaps we should postpone further experiments in multiethnicity until we understand more about this.