When is price discrimination profitable?

We consider a general model of monopoly price discrimination and characterize the conditions under which price discrimination is and is not profitable. We show that an important condition for profitable price discrimination is that the percentage change in surplus (i.e., consumers' total willingness to pay, less the firm's costs) associated with a product upgrade is increasing in consumers' willingness to pay. We refer to this as an increasing percentage differences condition and relate it to many known results in the marketing, economics, and operations management literatures.