So, over the past decade in particular but beginning some time before that, there has been an increased interest in analytical political philosophy in global distributive justice, roughly, whether and to what extent citizens of one state have obligations to provide certain material goods to those of another. Michael Blake's summary of the debate here is, I think, rather good. One of the things that those who think that people have special distributive obligations to members of their nation or sub-national culture appeal to in support of that claim is that people think that they owe substanially more to felloow members of such groups than to outsiders. Now, obviously, isolated facts about what people think they owe to others are never likely to settle a dispute about what people do owe to others - apart from anything else, as the Bernard Williams criticism of relativism, that it is either too early or too late, is meant to point out, appeal to agreement over some topic once there is a dispute about it is hardly likely to decide the disagreement. Further, metaethical, arguments are needed, and are often supplied, although I am rather sceptical about many of them.
What I am interested in here is the quasi-empirical claim which believers in the intrinsic moral importance of group membership appeal to, that the felt importance of group membership makes it easier to sustain relatively egalitarian distributive policies. Setting aside the philosophical issue of what is so great about relatively egalitarian distributive policies when one is arguing from within a perspective in which group membership is assigned special importance, the thought is that, granting that such policies would be desirable, since such group membership makes them easier to sustain, such group membership is desirable and morally important. Now, I think there is a problem for liberals making this argument, because liberals should be wary of assigning high levels of moral significance to maintaining ascriptive group identities like those of nationality, and I think that we can sort of see why if we think about the plausibility of the empirical claim that such group identities make it easier to sustain relatively egalitarian distributive policies. Indeed, I think that in some cases, what is significant about people's apparent concern for members of their own nation is not concern for members of their own nation as such, but simply happens to be a concern for another, non-ascriptive group, which happens to overlap with their nation.
The kinds of examples that get appealed to in this context are usually the Scandinavian countries; they are supposed to show that social democrat style polities can only be created relatively culturally homogeneous societies. Now, I don't really know very much about the various Scandinavian national identities, or indeed really about national identities in general - this has never stopped me in the past, though, so... An interesting contrast to Sweden and whatever cohesion is provided by the thick Swedish national identity, I think, might be provided by the States. For a Briton, at least, Americans seem to be aggressively patriotic - flags everywhere and so on, a kind of unreflective pride in and insistence in the general greatness of their nation. No-one, though, would claim that by European standards America has particularly egalitarian distributive policies, and indeed, a national identity which plays an important role in American citizens' lives seems to make it easier to stigmatise groups and so fail to meet what an egalitarian would regard as basic obligations to them. One only has to think of what was revealed in the aftermath of Hurricane Katrina, the way that race, amongst other things, had so clearly inflected policies both before and after its impact on New Orleans and the surrounding area.
This, though, makes a kind of sense if you think about how national identities work. National identities are exclusive; people either are or are not members of the nation. For lots of (white) Americans, it would seem that poor blacks, regardless of where they happen to live, are not part of the same nation as them. More than that, more than the bare fact of exclusion, and more worringly, exclusion, because nations are after all imagined communities, takes place on the basis of ascriptive characteristics, quasi-moralised features that decide whether one is or is not a member of the relevant group. Failure to live up to the standards demanded for membership becomes a failure to live up to the standards for even minimal requirements of moral consideration. Think of the outright racism of much of the American right's discussion of what to do about the Middle East; the frequency of the usually ill-disguised assertion that violence, in a variety of forms, is the only language Arabs understand. Or the way that criticisms of American policy are described as un-American, rather than wrong for some other, more universalist, reason.
Thick group identities create moralised in- and out-groups, and when such identities decide who is and who is not the subject of justice, then there is a problem, and a problem even within the ostensible reach of the group identity, because such a thick identity can have rather demanding conditions of membership, and failing to meet them has serious consequences. Maybe the Scandinavians have a different kind of identity to that of Americans, more inclusive, less politicised, or maybe it has not yet faced serious problems about dealing with potential non-members within the territory of Scandinavian states. There is of course another way of reading the concern for members of the same state, which, in most cases, overlaps with membership of the same nation; that it is a special concern for those who live under the same set of life-shaping coercive institutions, which, of course, are partly formed by some of the cultural features which pick out national groups. That, though, is a concern about autonomy, not ascriptive group membership.
Update: I swear, I wrote this before I saw this, but I quite agree with Jamie that the LRRS provides an excellent vehicle for expressing discontent at government attempts to get us to conform to some hackneyed, historically inaccurate piece of political expediency dressed up as a celebration of a national culture whose political expressions, like those of most national cultures, have generally been reactionary and bloody.