In theory, free entry can lead to social inefficiency. We study the radio industry in a first attempt to quantify this inefficiency. Using cross-sectional data on advertising prices, the number of stations, and radio listening, we estimate the parameters of listeners' decisions and of firms'...

Theory predicts that in markets with increasing returns, the number of differentiated products, and the tendency to consume, will grow in market size. I document this phenomenon across 247 U.S. radio markets. By a mechanism that I term "preference externalities," an increase in the size of the...