Top 40 Democracy: The Rival Mainstreams of American Music

If you drive into any American city with the car stereo blasting, you'll undoubtedly find radio stations representing R&B/hip-hop, country, Top 40, adult contemporary, rock, and Latin, each playing hit after hit within that musical format. American music has created an array of rival mainstreams, complete with charts in multiple categories. Love it or hate it, the world that radio made has steered popular music and provided the soundtrack of American life for more than half a century.

In T...