After World War II the popularity of the big bands collapsed suddenly. They were too expensive to run; furthermore, styles in entertainment had changed, and the smooth, high-powered band sound struck people as cold and slick. The mass market turned to rock’n’roll, itself the outcome of a vital new genre of African American music, rhythm and blues (see page 403). Within jazz itself, the collapse of the big bands had been foreshadowed during the war by a revolutionary new movement called bebop.