“The people of the Settlements select themselves. They select people like themselves. There’s a shared culture, even, to some extent, a shared physical appearance on each Settlement. Earth, on the other hand, is, and through all of history has been, a wild mixture of cultures, all enriching each other, competing with each other, suspicious of each other. Tanayama and many other Earthmen– myself, for instance–consider such a mixture to be a source of strength, and feel that cultural homogeneity on the Settlements weakens them and, in the long run, shortens their potential life span.”
From the book Nemesis by Asimov, page 123.