1 Highlights We study the impact of horizontal mergers on firms' critical discount factors The simulation setting is based on a random coefficient model of demand Results show that mergers strengthen the incentives to collude of the merged firm Results show that mergers weaken the incentives to collude of non-merging parties We propose screening tools able to approximate the magnitude of coordinated effects Abstract This paper aims at evaluating the coordinated effects of horizontal mergers by simulating their impact on firms' critical discount factors. We consider a random coefficient model on the demand side and heterogeneous price-setting firms on the supply side. Results suggest that mergers strengthen the incentives to collude among merging parties, but weaken the incentives of non-merging parties, with the former effect being stronger. To assess the magnitudes of these effects, we introduce the concepts of Asymmetry in Payoffs and Change in Payoffs effects, which allow us to identify appropriate screening tools according to the relative pre-merger payoffs of merging parties. a Corresponding author. Manufacture des Tabacs (Office MF 415) -21 Allée de Brienne,