Partisan divide creates different Americas, separate lives

When people try to explain why the United States is so politically polarized now, they frequently refer to the concept of "echo chambers."

That's the idea that people on social media interact only with like-minded people, reinforcing each other's beliefs. When people don't encounter competing ideas, the argument goes, they become less willing to cooperate with political opponents.

The problem goes beyond the online world. In my new book, " Overdoing Democracy: Why We Must Put Politics in its Place," I explain that in the United States, liberals and conservatives...