The obvious moral objection is that comparing slavery and immigration restrictions is absurd hyperbole. But it's absurd hyperbole to call this apt comparison "absurd hyperbole." Yes, enslaving a Haitian is plainly worse than forbidding him to accept a job offer anywhere on earth except Haiti. But they're both dire harms. How would you react if the world's laws barred you from every non-Haitian labor market on earth? With weeping and gnashing of teeth.Paradoxically, you might say, it's precisely Caplan's doctrinaire attitude that gradually made me reconsider the facts and adopt my present position. This corresponds with what he says further down: "Moderates are better at bargaining with people holding preferences fixed. Abolitionists are better at changing preferences." It also helps when the abolitionist is a highly well-informed economist.
I'm not sure why Caplan uses Haiti as his illustrative example every time he talks about the issue, but he really can get the point across. Immigration restrictions constitute one of the gravest injustices of our time, one which reinforces global social hierarchies--the poor remain poor while the rich remain rich--for no good reasons that cannot be addressed in other, more humane ways. This essay has pretty good answers to all the standard objections, particularly the ones which come from the right of the political spectrum.
Common political wisdom would suggest that the left is more naturally receptive to immigration, but I'm not so convinced. My guess is that in the long run, the left will have difficulty with truly open borders, precisely because of the difficulties presented in running a welfare state while letting everyone in (a classical right-wing objection to open immigration, to which the libertarian can reply, "Oh, well!"). And if what Caplan reports about the inverse relationship between diversity and redistribution is true, good old European socialists may be the last to embrace open borders.
As for me, I've come to see this as a plain implication of the existence of universal human rights. It comes down to a really fundamental question about the purpose of democratic governments. I don't believe that we can prohibit the movement of foreigners--of individual human beings who happen not to be born in our country--in the name of democracy, that is in the name of protecting our own interests. Indeed, when a country excludes certain people from immigration, it doesn't protect anyone's property, it only forces people not to exchange, and therefore not to mutually benefit from one another. For each immigrant you deport for trying to work in the U.S., you also deprive an employer of an employee he wanted to hire.
And this is a moral issue. If it were only a matter of economics, I think the empirical evidence might still make me an open borders libertarian, but the issue is far more urgent than that. There are people all around the world who could make a better life for themselves and their families if they had the freedom to move somewhere else. We need to let that happen. The first step is being convinced if the desirability of that goal.
I haven't thought what the next step might be, but I'm sure that will come.