30 years ago, two random Americans were twice as likely to have the same background

ESRI puts out a diversity index for the USA.  "0" means any two individuals picked at random  have the same race or ethnic background.  "100" means the opposite. Nowadays the average number for the USA is 65.  In 1980 is was 34.  The yellow areas have homogeneous populations that will view politics and change differently than the dark blue areas.  Areas with heavy Hispanic immigration are quite blue but so is the sparsely-settled north slope of Alaska with oil workers and Eskimos.

ESRI specializes in data overlays onto geographic maps.
Thanks to Small Dead Animals for alerting me to the existence of the index.
Below is a glimpse of the 2010 updated Diversity Index. ESRI report here.