According to the national identity argument, the sharing of a national identity tends to facilitate social cohesion, including trust and solidarity. On this basis, nationalists have, for example, argued that states’ immigration and integration policies need to reflect a concern with their national identities and the forms of homogeneity on which they depend. First, the national identity argument is outlined in greater detail as is the concept of social cohesion. Second, different possible explanations of why shared identities might impact social cohesion are described. Third, the national identity argument is explained in terms of the commitments of nationalism, from which it derives. Fourth, this paves the way for a survey of the empirical evidence that may support or undermine the national identity argument. It is argued that this argument does not find support in the available empirical studies. Finally, it is briefly considered whether there are other identities, apart from national identities, that may contribute to social cohesion at the societal level. In particular, liberal and multicultural values are considered.