  This argument can be contested on either empirical or moral grounds. For instance, without questioning whether restricting immigration is in some ways inefficient, one might still defend the merits of dividing the globe into separate states, each of which has sovereign control over its territory. This view seems plausible if the advantages to such a state system outweigh the inefficiencies in question. For instance, David Miller (2005) argues that restrictions on immigration improve our ability effectively to address population control. As Miller recognizes, we will make progress in this regard only if leaders in countries like India and China are able and willing to enact the relatively unpopular laws necessary to limit population growth. And these laws would be met with much more resistance than they already are if there were no restrictions on migration, because the costs of population growth would not be wholly internalized. If a country’s exploding population were free to move anywhere in the world, for instance, then its citizens would not bear the full costs of population growth and would therefore be that much more resistant to laws designed to limit the number of children born. If Miller is right, the advantages of restricting immigration may more than compensate for the costs involved.