# Ohio v. American Express (2018) **Argued:** February 26, 2018 · **Decided:** June 25, 2018 · **5–4, Justice Clarence Thomas** · 585 U.S. 529 The decision that wrote platform economics into antitrust doctrine, and the reason every subsequent Big Tech case has been fought first over market definition. Eleven states and the federal government challenged American Express's **antisteering rules**, which forbid merchants from encouraging customers to use a cheaper card. Amex charges merchants higher fees than Visa or Mastercard and returns much of it to cardholders as rewards; antisteering protects that model by preventing merchants from pushing back at the register. ## The holding Credit-card networks are **two-sided transaction platforms** — they cannot sell to one side without simultaneously selling to the other: > "The key feature of transaction platforms is that they cannot make a sale to one side of the platform without simultaneously making a sale to the other." Therefore the relevant market must include both merchants and cardholders as a single market, and a plaintiff must show net anticompetitive harm across the whole platform. Higher merchant fees alone prove nothing if cardholders receive offsetting value. On that framing the plaintiffs failed at step one of the [[Rule of Reason]] — they never got to the burden-shifting stage at all. Justice Breyer, dissenting with Ginsburg, Sotomayor and Kagan, called grouping complementary goods into one market "economic nonsense." ## The burden problem The criticism — made by the dissent and by a large subsequent literature — is that the majority front-loaded into the plaintiff's prima facie case what belongs in the defendant's rebuttal. A plaintiff must now negate the defendant's procompetitive justification on the other side of the platform *before* any burden shifts. Because platform-wide net-effects proof is expensive and often unattainable, the practical effect is to make dominant two-sided platforms very hard to sue. ## Where it landed *Amex* is on the 2020 House Judiciary report's list of decisions Congress should override, alongside *[[Verizon v. Trinko]]* and *[[Brooke Group v. Brown & Williamson]]*. Congress did not act. It then shaped the outcomes directly. Judge Boasberg's November 2025 judgment for Meta in [[FTC v. Meta]] turned entirely on market definition — that "personal social networking services" must include TikTok and [[YouTube]]. Judge Mehta's liability finding in [[United States v. Google]] survived because DOJ pleaded exclusive contracts rather than a two-sided pricing theory. **In platform antitrust, the market definition fight now is the case.** See [[US Antitrust and Monopoly Timeline]].