United States antitrust law
The rule of reason is a legal doctrine used to interpret the Sherman Antitrust Act, one of the cornerstones of United States antitrust law. While some actions like price-fixing are considered illegal "per se"", "other actions, such as possession of a monopoly, must be analyzed under the rule of reason and are only considered illegal when their effect is to unreasonably" "restrain trade. William Howard Taft, then Chief Judge of the Sixth Circuit Court of Appeals, first developed the doctrine in a ruling on "Addyston Pipe and Steel Co. v. United States, "which was affirmed in 1899 by the Supreme Court. The doctrine also played a major role in the 1911 Supreme Court case "Standard Oil Company of New Jersey v. United States."