Open borders advocates often advance an argument in terms of a duty to help the global poor. Poor people who succeed in making the journey to more advanced economies are usually more productive; those who are locked out of such economies by hard border controls are kept in dire poverty, often within sight of great riches. And those who are admitted are often an important source of income to family left behind. Those who defend border controls and the right of states to exclude often make the following move: they concede a duty to help the poor, but say that such a duty can be discharged in ways other than admitting poor would-be migrants to wealthy countries. In particular, they argue that such a duty could be discharged by supporting the economic development of poor countries via development aid (Christopher Heath Wellman is an example).
But the problem with such an argument is that it has two parts. The first (conditional) part, says that it is false that we must open our borders to discharge our duty of assistance IF we can discharge that duty some other way. The second empirical part is the claim there is another way, because development aid is an effective way of helping the global poor that is comparable in its beneficial effects to (much more) open borders. In other words, the claim by philosophers and political theorists that the duty could be discharged by development aid needs to be backed up by sound economic evidence that development aid really is an effective means of helping the global poor. Economists such as William Easterly are skeptical that we know enough about economic development to make effective use of development aid. They may be wrong, but philosophers and political theorists shouldn’t make the easy argumentative move to development aid as an alternative to (more) open borders without being sure that the economics supports them.