Tyler Cowen argues that allowing private defense agencies would not necessarily prevent a monopoly on defense services, by positing that a cooperating network of such firms could use aggressive force to enforce the cartel's market domination. Noting that advocates of PDAs typically argue that abuses would be prevented by the presence of rival agencies acting under the authority of rulings made by arbitrators empowered by inter-agency arbitration agreements, Cowen opines, "The adjudication network is stable only if it can use force to put down outlaw agencies that do not accept its higher-order arbitration decisions. Such a network could also use force to put down firms that do not adhere to the collusive agreement."