A colleague, Bruce Blonigen, has a new paper (coauthored with Justin R. Pierce):

Evidence for the Effects of Mergers on Market Power and Efficiency, NBER Working Paper No. 22750 Issued in October 2016: Study of the impact of mergers and acquisitions (M&As) on productivity and market power has been complicated by the difficulty of separating these two effects. We use newly-developed techniques to separately estimate productivity and markups across a wide range of industries using detailed plant-level data. Employing a difference-in-differences framework, we find that M&As are associated with increases in average markups, but find little evidence for effects on plant-level productivity. We also examine whether M&As increase efficiency through reallocation of production to more efficient plants or through reductions in administrative operations, but again find little evidence for these channels, on average. The results are robust to a range of approaches to address the endogeneity of firms’ merger decisions. [Open link to paper]