For a more extreme example, take Luzerne County surrounding Wilkes-Barre, Pa. Obama beat Romney there by 51.5 to 46.7. Four years later, Trump beat Clinton by a landslide margin, 57.9 to 38.6. In 2000, Luzerne had a diversity index of 8; by 2015 the index rose to 36.6. That’s a 360 percent increase over 15 years.

The same pattern can be seen in many of the Midwestern counties where the shift to Trump was strongest. Adams County, Wisconsin, voted for Obama over Romney by 53.9 to 45.1. Last year it backed Trump 58.9 to 37. From 2000 to 2015, the diversity index for Adams County rose from 7 to 21.3, a 204 percent increase. Macomb County, Michigan: 51.3 to 47.3 for Obama; 53.6 to 42 for Trump — its diversity index jumped from 16 in 2000 to 34.5 in 2015, a 116 percent increase.

At the same time, Wisconsin, Michigan, Ohio and, to a lesser extent, Pennsylvania, have each experienced a net out-migration of native-born residents. The effect of this exodus is twofold.