# Rule of Reason The interpretive move that converted the [[Sherman Antitrust Act]] from a categorical prohibition into a balancing test. §1 declares "every" contract in restraint of trade illegal; in the [[Standard Oil Breakup]] decision (1911), Chief Justice Edward White read in the word **"unduly"** — only restraints that unreasonably restrain trade are unlawful. Justice Harlan, concurring in the breakup but dissenting on the reasoning, called it judicial legislation. The alternative is the **per se rule**: certain conduct is unlawful without inquiry into effects, because experience shows it is almost always harmful. Price fixing, bid rigging, market allocation, and horizontal group boycotts remain per se illegal. Everything else has migrated, one category at a time, into the rule of reason — non-price vertical restraints in [[Continental TV v. GTE Sylvania]] (1977), maximum resale prices in *State Oil v. Khan* (1997), minimum resale prices in [[Leegin v. PSKS]] (2007). That migration is the quiet mechanism of the [[Consumer Welfare Standard]] era. A per se rule is cheap for plaintiffs to invoke; the rule of reason requires expert economics, market definition, and years of discovery. **Moving a practice from per se to rule of reason does not legalize it — it prices enforcement out of reach**, which for most plaintiffs is the same thing. ## Where the line is now The Roberts Court finished the migration. *Ohio v. American Express* (2018, 5–4) held that a "two-sided platform" must be analyzed as a single market — so a plaintiff must show net harm across both sides before the burden ever shifts — which is the doctrine every platform defendant now invokes. *NCAA v. Alston* (2021, 9–0) went the other way on the facts but confirmed the method: even the NCAA's amateurism rules get the full rule of reason, and Justice Kavanaugh's concurrence warned that under it they would lose. The [[New Brandeis Movement]]'s answer, written into the 2023 [[Merger Guidelines]], was not to abolish the rule of reason but to move the *presumptions* — structural thresholds that shift the burden to the defendant early — which is how the [[Sherman Antitrust Act]]'s categorical text is being reasserted without overruling a century of cases. Compare [[The Antitrust Paradox]], where [[Robert Bork]] argued that the rule of reason *is* the consumer welfare standard, and the [[US Antitrust and Monopoly Timeline]] for the sequence.