Internal versus External Growth in Industries with Scale Economics: A Computational Model of Optimal Merger Policy, August 8, 2018 version (FORTHCOMING)

We study optimal merger policy in a dynamic model in which the presence of scale economies implies that firms can reduce costs through either internal investment in building capital or through mergers. The model, which we solve computationally, allows firms to invest or propose mergers according to the relative profitability of these strategies. An antitrust authority is able to block mergers at some cost. We examine the optimal policy for an antitrust authority who cannot commit to its future policy rule and approves or rejects mergers as they are proposed, considering both consumer value and aggregate value as possible objectives of the antitrust authority. We also examine how the ability to commit would affect the antitrust authority's optimal policy and the welfare level it attains. We find that optimal policy can differ substantially from what would be best considering only welfare in the period the merger is proposed. We also find that the ability to commit can lead to a significant welfare improvement. In general, antitrust policy can greatly affect firms' optimal investment behavior, and firms' investment behavior can in turn greatly affect the antitrust authority's optimal policy.