On the social welfare effects of runner-up mergers in concentrated markets

This paper argues that it cannot be taken for granted that any merger that raises consumer surplus also increases social welfare. We assume a Cournot model with homogeneous goods, linear demand, and constant marginal costs, to show that a merger can raise consumer surplus while harming social welfare. Within this framework, we show that such an outcome depends on two conditions: the merger is between small firms (i.e., relatively inefficient firms) and it reduces concentration; that is, a constellation which can be characterized as a "runner-up" merger.