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 market brings forth additional products valued by others with similar tastes. But who benefits whom ? I document sharp differences in preferences between black and white, and between Hispanic and non-Hispanic, radio listeners. As a result, preference externalities are large and positive within groups, and they are much smaller and nonmonotonic across groups.