Research Handbook on the Economics of Antitrust Law
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Research Handbook on the Economics of Antitrust Law

Edited by Einer R. Elhauge

One might mistakenly think that the long tradition of economic analysis in antitrust law would mean there is little new to say. Yet the field is surprisingly dynamic and changing. The specially commissioned chapters in this landmark volume offer a rigorous analysis of the field’s most current and contentious issues.
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Chapter 3: Lightening Up on Market Definition

David S. Evans


David S. Evans* I INTRODUCTION ‘Market definition’ refers to the process of determining the set of products, and locations from which those products are sold, that are relevant for analysing the antitrust issue at hand. That set of products and locations defines ‘the market’. Courts have long treated market definition as the first step in analysing an antitrust matter.1 Among other things they rely on the relevant antitrust market to calculate market shares from which they infer the existence of market power. At least since Alcoa,2 the courts have drawn hard market boundaries. A product is either in or out of the market. The placement of this fence often determines the final outcome of the matter.3 As a result, market definition sets up a battle between the ‘we-win because it is a narrow market’ plaintiffs and the ‘you-lose because it is a broad market’ defendants.4 Both sides naturally invest significant resources in trying to persuade the courts where to build the fence. Many economists have argued that there is seldom a solid market boundary in practice.5 Products from different vendors are often heterogeneous and compete along a continuum. Economists have also observed that there is no particular need to define a rigid boundary. Ultimately antitrust is about ascertaining effects on prices, output, and other factors that influence consumer welfare. It is possible to address those effects directly without taking a firm position on a market boundary. In recent years, the US Department of Justice (DOJ) and the Federal Trade...

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