Areeda-Turner in Two-Sided Markets

We extend the Areeda-Turner rule to two-sided markets. We show that a two-sided monopolist may find it short-run profit-maximizing to charge a price below marginal cost on one side of the market. Hence showing that the price is below marginal cost on one side of a two-sided market cannot be considered a sign of predation. We then argue for a two-sided Areeda-Turner rule that takes into account price-cost margins on both sides of the market. Two examples highlight that applying a one-sided Areeda-Turner rule may lead one to assess legitimate prices as predatory or to consider predatory prices as legitimate.