Chris Sagers, the James A. Thomas Professor of Law, wrote on ProMarket, the blog of the Stigler Center at the University of Chicago, on the Supreme Court’s decision on Monday in Ohio v. American Express. In that case, a five member majority found that “anti-steering” rules imposed by American Express on merchants who accept its cards do not violate antitrust law. Sagers considered the so-called “two-sided market” theory on which the Court founded its decision, and what its acceptance by the Court says about antitrust enforcement more generally.