Ex ante or Ex post? When the timing of merger assessment is up to the merging firms

We develop a framework in which the timing of the merger control is left to the merging firms’ discretion: before the completion of the merger (ex ante) or afterwards (ex post). We show that the choice of merger control timing by the firms always dominates the ex ante control in terms of expected consumer surplus. The choice of merger control timing also dominates the ex post control except if the expected merger outcome is very anti-competitive.