Since the Supreme Court's landmark 1963 decision in Philadelphia National Bank, antitrust challengers have mounted prima facie cases against horizontal mergers that rested on the level and increase in market concentration caused by the merger, with proponents of the merger then permitted to...

Has the United States economy become less competitive in recent decades? One might think so based on a body of research that has rapidly become influential for antitrust policy. We explain that the empirical evidence relating to concentration trends, markup trends, and the effects of mergers...