Migrants by plane and migrants by stork: can we refuse citizenship to one, but not the other?

States combine the routine refusal of citizenship to migrants with policies that grant newborns of citizens (or residents) full membership of society without questions asked. This paper asks what, if anything, can justify this differential treatment of the two types of newcomers. It explores arguments for differential treatment based on the differential environmental impact, different impact on the (political) culture of the society in question and differences between the positions of the newcomers themselves. I conclude that, although some justification for differential treatment exists, the case for it is weaker than one may expect and the grounds on which it can be justified are surprising and problematic.