Competing Ad Auctions

by Itai Ashlagi, Benjamin G. Edelman & Hoan Soo Lee

Overview — Joining ad platforms can attract substantial regulatory attention: In November 2008, the Department of Justice planned to file antitrust charges to stop the proposed Google-Yahoo transaction. More recently, in September 2009, the Department of Justice sought additional information from Microsoft and Yahoo about their proposed partnership. At first glance it might seem paradoxical to claim that the Google-Yahoo transaction is undesirable, for advertisers and for the economy as a whole, while the Microsoft-Yahoo transaction offers net benefits. But that conclusion is entirely possible. HBS professor Benjamin G. Edelman and doctoral candidates Itai Ashlagi and Hoan Soo Lee explore competition among ad platforms that offer search engine advertising services. In addition, the authors evaluate possible transactions among ad platforms—building tools to predict which transactions improve welfare and which impede it. Key concepts include:

  • Participation costs exist and matter, affecting bidders' decisions about which ad platforms to use, and changing the welfare consequences of mergers or joins among platforms.
  • By creating a joined ad platform of larger size than Microsoft or Yahoo alone, the transaction lets advertisers spread participation costs over a larger purchase, making it worthwhile for small to midsize advertisers to sign up with the joined Microsoft-Yahoo platform even though they do not use Microsoft or Yahoo separately.
  • Preventing a competing platform from attracting advertisers reduces the quality of that competing platform (fewer ads yielding an inferior match with users' searches), cuts that platform's revenue (impeding future investment), and generally hinders that platform's efforts at growth.

Author Abstract

We present a two-stage model of competing ad auctions. Search engines attract users via Cournot-style competition. Meanwhile, each advertiser must pay a participation cost to use each ad platform, and advertiser entry strategies are derived using symmetric Bayes-Nash equilibrium that lead to the VCG outcome of the ad auctions. Consistent with our model of participation costs, we find empirical evidence that multi-homing advertisers are larger than single-homing advertisers. We then link our model to search engine market conditions: we derive comparative statics on consumer choice parameters, presenting relationships between market share, quality, and user welfare. We also analyze the prospect of joining auctions to mitigate participation costs, and we characterize when such joins do and do not increase welfare.

Paper Information