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This article seeks an answer to a question that should be well settled: for purposes of antitrust analysis, what is 'market power' and/or 'monopoly power'? The question should be well settled because antitrust law requires proof of actual or likely market power or monopoly power to establish most types of antitrust violations.

Examination of key antitrust law opinions, however, shows that courts define 'market power' and 'monopoly power' in ways that are both vague and inconsistent. We conclude that the present level of confusion is unnecessary and results from two different but related errors:

(1) the belief or suspicion that market power and monopoly power are two different concepts, when they are in fact, for antitrust purposes, qualitatively identical. We argue that attempting to distinguish between market power and monopoly power creates a false dichotomy; and

(2) the failure to recognize that anticompetitive economic power can manifest itself in two distinct ways, and these differences have significant legal and policy implications. The true distinction is between anticompetitive economic power that is exercised by restricting one's own output, and such power exercised by restricting the output of one's rivals. Identifying this fundamental distinction and discarding the false one can help to clarify a number of troublesome antitrust issues.

The body of this article describes these conclusions, and the bases for them, in some detail. The appendix presents a shorter, more technical description of the principal argument. Readers already familiar with the main body of antitrust law and conversant with antitrust economics may wish to begin by reading the appendix.





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