Steven Landsburg compares how much open immigration costs low-wage American workers to how much it benefits the immigrants themselves, and reaches this, ah, debatable conclusion:
The most conservative standard assumption is that the value of an extra dollar is inversely proportional to your income, so an extra dollar is worth five times as much to a $2-an-hour Mexican as it is to a $10-an-hour American ... Accounting for all that, it turns out that the immigrant's $7 gain is worth about five times the American's $3 loss. In other words, to justify keeping the immigrant out, you'd have to say he's worth less than one-fifth of an American citizen.
By contrast, there was a time when the U.S. Constitution counted a black slave as three-fifths of a full-fledged citizen. Alabama Gov. Bob Riley has recently apologized for the ravages of slavery. How long till politicians apologize for the ravages of our restrictive immigration policies?
Actually, the U.S. Constitution only counted black slaves as three-fifths of a full-fledged citizen for the purposes of apportioning Congressmen; otherwise, it counted them as property or not at all. The problem with slavery was, well, slavery, not the three-fifths compromise, which was just a legal epiphenomena of the peculiar institution. Prior to abolition, slaves weren't legal persons; after abolition, they became Americans, hobbled by Jim Crow but technically entitled to all the benefits of citizenship. They never actually existed in the kind of three-fifths limbo that Landsburg conjures up.
Needless to say, the situation with Landsburg's hypothetical would-be migrant is completely different. Rather than a slave seeking legal acknowledgement of his humanity, you have a Mexican citizen seeking access to the financial benefits of American citizenship. The slaves lacked legal protections of any kind; our hypothetical Mexican lacks, well, access to better pay. What we're haggling about in the latter case, in other words, isn't the recognition of human rights, but to what extent a state should pursue policies that benefit foreign nationals at the expense of its own citizens.
I don't have a definite answer to this question. I wouldn't mind if our foreign aid budget were higher; on the other hand, I'm not at all bothered by the fact that our government spends far more on welfare programs at home than it does on humanitarian aid abroad, even though by Landsburg's argument it means that we're treating a Congolese refugee as one-seventieth of a human being. This seems to me self-evidently ridiculous, for the same reason that it would be ridiculous to claim that by spending $30,000 on a nursing home for my father and giving $30,000 to a crowded homeless shelter in the same year, I'm behaving as if the homeless people's lives are worth literally hundreds of times less than my father's. I have less of a responsibility for them, sure, but the fact that moral responsibilities vary, for governments as well as people, isn't a sign of our moral deficiency; it's a recognition of a basic principle that all of human society - yes, even Peter Singer's family - depends upon to flourish.