Accumulating evidence points to the need for more vigorous antitrust enforcement in the United States in three areas. First, stricter merger control is warranted in an economy where large, highly efficient and profitable “superstar” firms account for an increasing share of economic activity....

We describe a simple initial indicator of whether a proposed merger between rivals in a differentiated product industry is likely to raise prices through unilateral effects. Our diagnostic calibrates upward pricing pressure (UPP) resulting from the merger, based on the price/cost margins of the...