We propose a framework for analyzing transformations of demand. Such transformations frequently stem from changes in the dispersion of consumers' valuations, which lead to rotations of the demand curve. In many settings, profits are a U-shaped function of dispersion. High dispersion is complemented by niche production, and low dispersion is complemented by mass-market supply. We investigate numerous applications, including product design; advertising, marketing and sales advice; and the construction of quality-differentiated product lines. We also suggest a new taxonomy of advertising, distinguishing between hype, which shifts demand, and real information, which rotates demand.

Firms selling multiple quality-differentiated products frequently alter their product lines when a competitor enters the market. We present a model of multiproduct monopoly and duopoly using a general “upgrades” approach that yields a powerful analytical framework. We provide an explanation for the common strategies of using “fighting brands” and of product line “pruning.” The optimal strategy depends on whether entry prompts an incumbent to expand or contract its total output. We also present a general condition that guarantees that a monopolist will sell but a single product. Our model addresses other issues, including intertemporal price discrimination and “damaged goods.”