On paper, Lexington County, S.C., and Placer County, Calif., have a lot in common. They’re both big, wealthy, suburban counties with white supermajorities that border on their respective state’s capital cities…But when it comes to how long their residents can count on living, the parallels fall apart. Placer has a Scandinavia-like life expectancy of 82.3 years. In Lexington, the figure is 77.7, a little worse than China’s. Step back and look at a map of life expectancy across the country and the geographic patterns are as dramatic as they are obvious. If you live pretty much anywhere in the contiguous...