Final answer:
The main idea is that post-war prosperity led to the rise of new mass culture.
Step-by-step explanation:
The main idea conveyed in the passage regarding the emergence of mass culture in the U.S. after the 1920s and 1950s.
During the 1920s, rising earnings and technological innovations resulted in the booming popularity of movies, sports, and radio programs. Advertising became a central institution in the consumer economy, and commercial radio and magazines influenced mass culture.
Similarly, after World War II, Americans had more disposable income and leisure time, leading to the growth of a consumer culture with the expansion of mass media like movies, radio, and television.