Where America's diversity is increasing the fastest

America is becoming more racially and ethnically diverse, on its way to becoming majority non-white in 2045 — but some parts of the country are changing more rapidly than the rest.

Why it matters: That's worth remembering as we celebrate July 4 at a time of accelerating changes and political upheaval. The counties seeing the greatest relative increase in racial and ethnic diversity are among the least diverse places in the country — particularly in the Midwest.

How we made this: Using Census data, we calculated a diversity index for every county in the United States going back to 2009. Each number represents the probability that two people chosen at random will be of a different race or ethnicity.

• The counties in the map are shaded by the percent change in the index value from 2009 to 2017.

The big picture: The country as a whole has a diversity score of 57.3, which means that there is about a 57 percent chance that two people chosen at random will be of a different background.

• Since 2009, the country's diversity index has increased about 5 points.

The most diverse counties tend to contain a big city or be near one. Queens County, N.Y. is the most populous (2.3m), most diverse (76.4) county we found using our measure.

Some notable exceptions:

• Miami-Dade County (49.9) is less diverse than the country as a whole, mainly because 70 percent of its population is Hispanic.
• Of all counties with more than 1 million people, the least diverse by far was Allegheny County (35.7), which is home to Pittsburgh.

The least diverse counties tend to be more rural, have smaller populations, and are farther from the coasts. In those places, a small influx of people can have a dramatic effect on its diversity score.

Population isn't everything. Hawaii County, which encompasses the state's Big Island, has the highest diversity score of all counties we analyzed (77.8), with a population of just 196,000.

• Some places are actually becoming less diverse — including Miami-Dade County as well as Hidalgo County, Texas (15.3), home of McAllen, Texas. Over 90 percent of Hidalgo's population is Hispanic.
• Many counties along the U.S. Mexico border in a similar situation: they're majority Hispanic and becoming even more so, decreasing their diversity index.