Rev Ind Organ (2011) 39:69–93
John E. Lopatka
Published online: 22 June 2011
© Springer Science+Business Media, LLC. 2011
Abstract The 2010 horizontal merger guidelines issued by the antitrust agencies de-
emphasize market definition, which has been a foundation of merger law for decades
and was required by previous guidelines. The justiﬁcation for this change is that unilat-
eral adverse price effects caused by mergers of ﬁrms that produce differentiated prod-
ucts are best assessed using analyses that do not depend on market definition. Though
the guidelines unquestionably serve a value in illuminating actual agency practice, any
marginal beneﬁt in dropping the exercise of market determination from the merger
review process is likely to be small, and the marginal cost may be substantial.
Keywords Antitrust · Market definition · Merger · Merger guidelines ·
Merger analysis requires the definition of a market. Or so we thought. For decades,
the market definition aphorism has been understood to be a part of antitrust dogma.
Most courts do not even bother stating it. Indeed, courts often instruct that merger
analysis begins with the definition of a market. Commentators have recognized the
need to deﬁne a market, and for their part, the enforcement agencies have, too.
Beginning in 1968 through four sets of horizontal merger guidelines and one ofﬁcial
commentary, the Department of Justice (DOJ) has acknowledged the central role of
For an informative summary of recent case law requiring market definition, see Brannon and Bradish
J. E. Lopatka (
Dickinson School of Law, Pennsylvania State University, 337 Katz Building, University Park,
PA 16802, USA