Contrôles a priori et a posteriori des concentrations : comment augmenter l'efficacité des politiques de concurrence

A recent French merger case involving two large water companies seems to toll the death of ex-post merger control. This paper aims at highlighting the interest of that type of control by showing its crucial role for competition authorities as a complement of the current notification system. Stressing the limits of an exclusively ex-ante control, we show that the possibility to act ex-post on market structures would lead to more virtuous behaviors in the ex-ante control by reducing adverse selection effects. The ex-post merger control would also permit new means of action on agents? behaviors after the merger. Despite those positive effects, the implementation of an ex-post merger control should be attentively carried out in particular because some legal security problems could emerge. However, those issues might be offset by increasing the competence of the French competition authority in merger cases.