How Merger Synergies Can Harm Consumers: A Defense of the Efficiencies Offense

This paper uses an aggregative games framework to predict consumer welfare when market structure is endogenously determined. Our main results characterize mergers whose synergies reduce consumer welfare by inducing rivals to exit. The conditions under which such mergers arise are broad, regardless of whether we consider quantity competition among homogeneous products or price competition among multi-product firms facing multinomial logit demand. Calibrated models based on commonly used parameter values indicate that the synergies required to avoid consumer harm can be much higher than those implied by traditional merger analysis. Neither subsequent entry nor follow-on mergers necessarily mitigate the problem.