We - re used to thinking of the United States in opposing terms: red versus blue, haves versus have-nots. But today there are three Americas. At one extreme are the brain hubs - cities like San Francisco, Boston, and Durham ...

We - re used to thinking of the United States in opposing terms: red versus blue, haves versus have-nots. But today there are three Americas. At one extreme are the brain hubs - cities like San Francisco, Boston, and Durham ...