Concentration-based thresholds for horizontal mergers, such as those in the US Horizontal Merger Guidelines, play a central role in merger analysis but their basis remains unclear. We show that there is both a theoretical and an empirical basis for focusing solely on the change in concentration, and ignoring its level, in screening mergers for whether their unilateral price effects will harm consumers. We also argue that current threshold levels likely are too lax, unless one expects efficiency gains of 5 percent or greater, or other factors such as entry and product repositioning to significantly constrain the exercise of market power postmerger.
We develop a theory of collective brand reputation for markets in which product quality is jointly determined by local and global players. In a repeated game of imperfect public monitoring, we model collective branding as an aggregation of quality signals generated in different markets. Such aggregation yields a beneficial informativeness effect for incentivizing the global player. It however also induces harmful free-riding by local, market-specific players. The resulting trade-off yields a theory of optimal brand size and revenue sharing that applies to platform markets, franchising, licensing, umbrella branding, and firms with team production.