Collusion at the extensive margin

We augment the multi-market collusion model of Bernheim and Whinston (1990) by allowing for firm entry into, and exit from, individual markets. We show that this gives rise to a new mechanism by which a cartel can sustain a collusive agreement: Collusion at the extensive margin whereby firms collude by avoiding entry into each other's markets or territories. We characterise parameter values that sustain this type of collusion and identify the assumptions where this collusion is more likely to hold than its intensive margin counterpart. Specifically, it is demonstrated that where duopoly competition is fierce collusion at the extensive margin is always sustainable. Finally, we provide a theoretic foundation for the use of a "proportional response" enforcement mechanism.