source: http://exn.ca/Stories/1997/04/01/03.asp - an article on the report by Dr. Jorde published in the Proceedings of the National Academy of Science.

"The scientists measured the genetic diversity among three racial groups - 65 Asians, 72 Africans and 120 Europeans - by counting the number of mutations or changes found in their mitochondrial DNA. Jorde found the greatest diversity among Africans, a strong indication that they are the ‘oldest' population."

it isn't "bad", it just isnt "optimal" for a group to promote unions with outsiders, those engaged in such "mixed" relationships and the children of "mixed" backgrounds tend to have divided loyalties, and a confused outlook in terms of their "place"/role within either community even IF they are embraced by one or both sides, they tend to lack a solid and innate understanding of either group.

it just isnt positively affirming for the culture if its members are forming their most meaningful relationships (husband/wife) with outsiders, the end result is the death of a culture, as it gets swallowed up and assimilated by others.