Efficiencies,market power and horizontal merger

In June 1982 the Justice Department issued itsMerger Guidelines which specify in terms of the Herfindahl index (H) what combinations of merger size and post-merger H are likely to lead to a merger challenge. This paper assesses theseGuidelines using Williamson's (1968) well-known model in which an optimal merger policy is viewed as one that considers both the price and cost consequences of merger. The Williamson model is recast in terms of H and changes in H and linked to theGuidelines. This allows an assessment of the welfare congequences of an industry merger for any given level of concentration and merger-produce changes in concentration. Among the conclusions are that, consistent with theGuidelines, higher values of H make socially successful mergers less likely, and a more appropriate, if perhaps not more feasible, focus for theGuidelines are coordination adjusted measures of concentration and merger size.