Harold Demsetz of UCLA suggested controlling for market share . What he found is that the market leaders in each industry tend to have above-average profits, but concentrated industries overall have average profits. While Demsetz provides some evidence for market monopoly based on superior efficiency, there's very little evidence that any other kind of monopoly exists on the market. To answer this, we must first investigate what antitrust law purports to do, and what it actually does. Simply setting high prices almost never constitutes an antitrust violation.
Treble damages makes suing very attractive.
Enforced break-ups rare, especially since 1950. The effort to impose perfect competition on industries with a big minimum efficient scale. Price discrimination can completely eliminate all allocative inefficiency from monopoly. Superficially, there seems to be a lot of price discrimination. Ban on price-fixing and collusion Impedes research joint ventures, joint use of patents, etc.