ABSTRACT The decision of the majority in the US Supreme Court in Ohio v American Express appeared to provide a ‘blueprint’ for defining markets when analysing the effects of conduct engaged in by digital platform businesses. It was suggested that to analyse conduct engaged in by a transaction platform, a single market (that includes customers on both sides of the platform) should be identified, while for non-transaction platforms, single-sided markets should be identified. Others have argued that this approach is problematic. We agree that it is not as simple as classifying the business as a transaction or non-transaction platform and then defining one or multiple markets (respectively). The purposive approach to market definition suggests that greater emphasis needs to be placed on the impugned conduct and where its effects will be felt, rather than on the nature of the business engaging in said conduct. This article therefore posits a different approach and develops an argument that attention should be focused on whether the effect of the conduct is horizontal (that is, competition is affected at the platform level) or vertical (that is, the conduct impacts competition in relation to one or more of the platform's customer groups).
Smith et al. (Tue,) studied this question.