 Questions arise, however, about the extent to which any domain of international law can achieve legitimacy in these terms, especially given the absence of reliable enforcement mechanisms at the global level. This is because, on the NJC, one way in which legitimacy can be acquired is through the capacity of legal institutions effectively to impose solutions to various societal problems—such as co-ordination problems regarding environmental protection—through its capacity to enforce the standards it lays down. This reveals the familiar substantive dependence of de jure legitimacy on de facto legitimacy: the fact that the subjects of law are likely to comply with it enhances its legitimacy. Some have questioned whether international law can claim any legitimacy given the absence of reliable enforcement mechanisms (Posner 2009). This is a problem that is especially acute in the area of international human rights law, since the usual expedient of a tit-for-tat response to another state’s defection from the law seems out of place here. It will hardly be an incentive for state X to comply with international human rights law if state Y threatens non-compliance in relation to its own citizens.