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Abstract

Since the Supreme Court's landmark 1963 decision in United States v. Philadelphia National Bank, antitrust challengers have mounted prima facie cases against horizontal mergers that rest on the level and increase in market concentration caused by the merger. Proponents of the merger are then permitted to rebut by providing evidence that the merger will not have the feared anticompetitive effects. Although the means of measuring that concentration as well as the triggering levels have changed over the last half century, this basic approach has remained intact. This longstanding structural presumption has been critical to effective merger enforcement. In this Feature, we argue that the structural presumption is strongly supported by economic theory and evidence and suggest some ways to further strengthen it. We also respond to those who would weaken or eliminate it. Our analysis applies to the modem legal landscape, where the promotion of competition and the protection of consumer welfare are considered the purpose of merger enforcement.

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