A lot of people here refrences Latin America, and I would also point to ashkenazi jews. In fact, because there was a flow of individuals in and out the jews did become smarter than Asians. They are also a good example of why we should try to avoid mixing. Jews didn't really belong anywhere so they spent a few millenia roaming europe and getting kicked out of places periodically. They did manage to maintain strong blood ties, but because that wasn't bound to any soil they didn't really create anything. Now that they have essentially become a race I am glad that they are settling down in Israel and hope them the best when they get there.

Americans are a mix of genetically similar people, but a mix none the less. Because our ancestors were similar it was easier to keep most of a common heritage and fill in the rest, and even then it only really worked because we stole land from indians. Without any new land a mixed race will cause conflict either within a country it doesn't belong in, or when it tries to make its own place.