Merger Analysis with IIA Demand

Using an aggregative games approach, we analyze horizontal mergers in a model of multiproduct-firm price competition with a demand system that satisfies the Independence of Irrelevant Alternatives (IIA) property. We show that the Herfindahl index provides an adequate measure of the oligopoly distortions to consumer surplus and aggregate surplus, and that the induced change in the naively computed Herfindahl index is a good approximation for the market power effect of a merger. We also provide conditions under which a merger raises consumer surplus, and conditions under which a myopic, consumer-surplus-based merger approval policy is dynamically optimal. Finally, we study the aggregate surplus and external effects of a merger.