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Antitrust is back on the national agenda. The Democratic Party, leading Senators, progressive organizations, and many scholars are calling for stronger antitrust enforcement. One important step, overlooked in the discussion to date, is to reform how market power — an essential element in most antitrust violations — is determined. At present the very definition of market power is unsettled. While there is widespread agreement that market power is the ability to raise price profitably above the competitive level, there is no consensus on how to determine the competitive level. Moreover, courts virtually never measure market power (or its larger variant, monopoly power) by identifying the competitive level and comparing a defendant’s price to it. Rather, they attempt to define a relevant market and calculate the defendant’s market share, a process that is often complex and misleading. This Article proposes a new approach that would infer market power from the likely effects of the challenged conduct. Courts ought to identify power by asking whether the challenged conduct is likely to enable the defendant(s) to raise price above the prevailing level or maintain price above the but for level (the level to which price would fall absent the challenged conduct). This method would not only close the definitional gap, it would enable courts to resolve two critical elements of most antitrust offenses — market power and anticompetitive effects — at the same time, while inferring the relevant market from the result. In short, by reducing the cost and improving the accuracy of antitrust enforcement, this step would increase its impact.

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