Antitrust ostensibly aims to keep markets competitive. Such regulation is justified, it is claimed, because markets too frequently become monopolized in the absence of government intervention to ensure competition. Using both civil and criminal sanctions, antitrust statutes seek to prevent the three classes of behavior, all thought to be anticompetitive and, thus, harmful to consumers: collusion among rivals, mergers that threaten to create excessive monopoly power, and any number of exclusionary practices— such as predatory pricing—that allegedly hamper rivals' ability to compete.

The longstanding and still-popular explanation for the enactment of antitrust legislation in the United States holds that, starting in the 19th century, Congress sought to protect consumers from the increasing monopolization of the American economy. Judge Richard Posner's summary statement of the origins of ...