Abstract: The recently issued revision of the U.S. Horizontal Merger Guidelines, like its
predecessors and mirrored by similar guidelines throughout the world, devotes substantial
attention to the market definition process and the implications of market shares in the market that
is selected. Nevertheless, some controversy concerning the revised Guidelines questions their
increased openness toward more direct, economically based methods of predicting the
competitive effects of mergers. This article suggests that, as a matter of economic logic, the
Guidelines revision can only be criticized for its timidity. Indeed, economic principles
unambiguously favor elimination of the market definition process altogether. Accordingly, the
2010 revision is best viewed as a moderate, incremental, pragmatic step toward rationality, its
caution being plausible only because of legal systems’ resistance to sharp change.