The “drowning child” is a hypothetical concocted by the ethicist and philosopher Peter Singer. A short version can be found in this article by Peter Singer for Open Democracy. A longer version is in his book The Life You Can Save: Acting Now To End World Poverty.

Imagine you come across a small child who has fallen into a pond and is in danger of drowning. You know that you can easily and safely rescue him, but you are wearing an expensive pair of shoes that will be ruined if you do. We all think it would be seriously wrong to walk on past the pond, leaving the child to drown, because you don’t want to have to buy a new pair of shoes – in fact, most people think that would be monstrous. You can’t compare a child’s life with a pair of shoes!

Singer then uses this to argue that well-off people have a moral obligation to donate money to save the lives of some of the world’s poorest people, as long as they can do this without sacrificing anything very significant.

Surprisingly, Singer does not make an argument for open borders. Economist Tyler Cowen asked him about it in a Bloggingheads video (link goes to relevant segment) (full transcript) and he said he hadn’t given the matter enough thought. This earned him a mention in a blog post by Michael Strong.

However, the drowning child parable can be used to make an even stronger case for open borders. In a blog post about the drowning child, Caplan writes:

Singer is quick to conclude that we have massive obligations to help millions of desperate strangers. But all First World countries violate a far less controversial obligation to the world’s poor. Under the guise of “immigration policy,” every First World country makes it a crime for foreigners to accept a job offer from a willing First World employer. Economically speaking, this is a big deal: The best evidence says that worldwide free trade in labor would double world GDP and vastly increase Third World wages.

We shouldn’t talk about the world’s poor like they’re Drowning Children. If we lived up to our basic obligation to leave strangers alone, the world’s poor wouldn’t need our charity.

In a subsequent blog post, Caplan argues that immigration restrictions are more like people deliberately trying to prevent other people from rescuing drowning children:

Reasonable people can argue about our moral obligations to Drowning Children. But telling would-be rescuers “You have to wait until you prove you don’t have nefarious intentions” is clearly wrong. When someone tries to rescue a Drowning Child, anyone who interferes has blood on his hands.