Modular Market Definition

52 Pages Posted: 12 Jan 2021

See all articles by Sean Sullivan

Sean Sullivan

University of Iowa College of Law

Date Written: November 25, 2020


Recent interest in curbing the apparent market dominance of companies like Amazon, Facebook, Apple, and Microsoft is exposing the public to a difficulty long known to the antitrust community: the challenge of deciding in what markets a company operates. If the same company that dominates a narrowly defined market would fade into the crowd in a more broadly defined market, then is antitrust intervention really warranted? Antitrust law has a long history with this question, but it is a history punctuated as much by setback as by success. Even today, antitrust litigation stumbles at market definition with shocking regularity.

This Article introduces modular market definition—a tool for more systematically and reliably defining markets in antitrust cases. Decoupling the purpose of market definition from the process of the exercise, modular market definition turns conflicting tests of market definition into a series of complementary modules, each appropriate for scoping markets in a particular context. This streamlines market definition by providing a principled guide to defining markets, even in complex situations. It also reveals defects in how antitrust tries to use market definition today.

Keywords: market definition; relevant market

JEL Classification: K21, L40

Suggested Citation

Sullivan, Sean, Modular Market Definition (November 25, 2020). Available at SSRN:

Sean Sullivan (Contact Author)

University of Iowa College of Law ( email )

Melrose and Byington
Iowa City, IA 52242
United States


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