Part of how we react to these changes as a country will be driven by what our communities looked like before them. And here, the decline of the all-white county is instructive. Surveys by the Public Religion Research institute show that nearly 40 percent of Americans say that newcomers from other countries threaten traditional American customs and values. Thirty one percent of white people say that "the idea of America where most people are not white bothers me."

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This discomfort with growing diversity is generally most evident among Americans who are older and more conservative. To understand it, it helps to understand where they're coming from. The census data show that roughly half of white Americans lived in a county where 9 out of 10 people were white in 1980. Today, just over a quarter do. Raising our threshold to 98 percent white, as in the maps above, shows that 15 percent of white Americans lived in these near-exclusively white areas in 1980, while less than one percent do today.

Of course, these thresholds are somewhat arbitrary. A county that goes from 98.1 percent white in 1980 to 97.6 percent white in 2010 is not necessarily experiencing a demographic revolution. But the sharp drop in the aggregate number of these nearly all-white counties makes for a useful marker of demographic change.

This 30-year timespan is within living memory for many of the people expressing unease with racial changes. They can look back on a time when literally everyone around them looked like them, and compare it to the present day, where people are becoming more and more different from each other -- at least at the level of skin color.

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It may partially explain -- although not excuse -- the notion among some whites that they are besiege. These changes may also animate some of the white resentment that periodically flares up in the aftermath of episodes like the killing of Michael Brown in Ferguson, Mo.