I would refer you to the Putnam study from another comment, which defines homogeneity/heterogeneity in terms of ethnicity.
Certainly ethnicity is a fluid concept. The language, value system, and cultural context of a Texan may differ from that of a Minnesotan. However, I think we can agree that your average Texan or Minnesotan will likely have a greater common understanding if paired together than with your average Swiss or German person.