Robin Mason University of Southampton and CEPR Helen Weeds University of Essex and CEPR 16 August 2007

We assess the impact of merger policy on entry and entrepreneurship. Facing uncertainty about its prospects and foreseeing that it may wish to quit should profitability prove poor, a rational entrant considers possible exit routes. Horizontal merger reduces competition subsequently, lowering welfare in the short run, but also provides a valuable exit route. By facilitating exit and thus raising the value of entry, more lenient merger policy may stimulate entry sufficiently that welfare is increased overall. We calculate the optimal merger policy in the form of a low, but positive, profitability threshold below which a merger is permitted despite its adverse impact on post-merger competition. This may be viewed as an extension of the “failing firm defence” to include ailing, low profitability firms as well as imminently failing ones. The implications of strategic firm behaviour for the optimal policy are examined, and merger policy is compared with an