The orthodox view of market definition in antitrust cases is that the same principles of market definition should apply at all stages of an antitrust analysis, and, in particular, that markets should be defined for virtually all purposes by reference to demand-side substitutability. Commentators have struggled to reconcile the Supreme Court’s recent decision in Ohio v. American Express Co. –in which the majority combined services to cardholders and services to merchants into a single antitrust market, despite the evident lack of substitutability between them — with that familiar view.
In this short Article, we suggest that this effort at reconciliation may be unnecessary, and perhaps even undesirable. Against the orthodox view, we claim that market definition should be “disaggregated” such that the correct approach to market definition may vary depending on the element of the antitrust analysis for which it is being used. Thus, while market definition based on substitutability is an appealing tool for the assessment of market power, it may not be appropriate for the evaluation of competitive effects in all cases under Section 1 of the Sherman Act. The majority opinion in AmEx can (and perhaps should) be understood as an implicit-albeit cryptic-endorsement of a disaggregated approach to market definition.