It has been suggested that mergers, by increasing concentration, raise incentives to invest and hence are pro-competitive. To study the effects of mergers, we rewrite a game with simultaneous price and cost-reducing investment choices as one where firms only choose prices, and make use of aggregative game theory. We find no support for that claim: absent efficiency gains, the merger lowers total investments and consumer surplus. Only if it entails sufficient efficiency gains, will it be pro-competitive. We also show there exist classes of models for which the results obtained with cost-reducing investments are equivalent to those with quality-enhancing investments.