Mergers of business firms violate the antitrust laws when they threaten to lessen competition, which generally refers to a price increase resulting from a reduction in output. However, a merger that threatens competition may also enable the post-merger firm to reduce its costs or improve its product. Attitudes toward mergers are heavily driven by assumptions about efficiency gains. If mergers of competitors never produced efficiency gains but simply reduced the number of competitors, a strong presumption against them would be warranted. We tolerate most mergers because of a background, highly generalized belief that most or at least many produce cost savings or improvements in products or service. This article considers the current approach...