If I could add one amendment to the Constitution, it would be the one Wall Street Journal editorial page editor Robert Bartley once proposed: "There shall be open borders." There is no single policy that the United States could adopt that would do more good for more people. An average Nigerian worker can increase his income almost 15-fold just by moving to the United States, and residents of significantly richer countries like Mexico can more than double their earnings. The humanitarian gains of letting everyone who wants to make that leap do so would be astounding.

So I was disappointed, if not surprised, at the visceral horror with which Bernie Sanders reacted to the idea when interviewed by my colleague Ezra Klein. "Open borders?" he interjected. "No, that's a Koch brothers proposal." The idea, he argued, is a right-wing scheme meant to flood the US with cheap labor and depress wages for native-born workers. "I think from a moral responsibility, we've got to work with the rest of the industrialized world to address the problems of international poverty," he conceded, "but you don't do that by making people in this country even poorer."

There are two problems with Sanders's view on this, one empirical and one moral. He's wrong about what the effects of an open-border policy would be on American workers, and he's wrong in treating Americans' lives as more valuable and worthy of concern than the lives of foreigners.

Open borders would make Americans richer, not poorer

The existing economic literature suggests that eliminating all barriers on movement between nations would increase world GDP by 50 to 150 percent. The midpoint estimate is that the world economy would double. That's because people are much more productive in rich countries. Because of better technology, more skilled co-workers, better institutions, and the like, a worker doing the same job will earn vastly more for it in the US than in, say, Haiti. And if everyone were able to take jobs where they'd earn the most, the cumulative effect on the economy would be massive.

Even the biggest opponents of immigration will concede that much. Immigration obviously increases growth, just as tearing down trade barriers does. The question is whom that growth goes toward. A lot of it goes to migrants, who see their incomes grow dramatically for doing the same work. Even according to George Borjas, the single most-cited anti-immigration economist, immigration doesn't make the existing workforce worse off on average. But it does, he claims, most likely reduce wages substantially for people lacking high school degrees.

There are a few things to say in response to this. One is that even if there are losers from immigration, it should be possible to compensate them by redistributing money from the winners. The second is that Borjas is only looking at relative effects: how high school dropouts are affected compared with, say, college graduates. He actually assumes that the effect on native workers as a whole is neutral. If the effect on all workers is positive, it's possible that the absolute effect on high school dropouts is positive, even if they gain less than other workers. It's also worth noting that immigration appears to boost high school graduation rates — so even if high school dropouts are made worse off, there would be fewer people bearing that burden.

A huge spike in Russian immigration to Israel in the early 1990s appeared to give existing workers a nearly 9 percent raise

The third point is that Borjas's results are heavily contested — and most of the rest of the literature suggests that the effect on native workers' wages is neutral or positive. In particular, high-quality studies that use "natural experiments" — cases where there was a big, unexpected spike in immigration — suggest that the absolute effect of immigration on native workers is neutral or positive. It's much easier to isolate the effect on native workers in those cases than it is by trying to statistically weed out other potential causes of changes in wages. The Mariel boatlift, when Cuba unexpectedly sent 125,000 people to Florida, did not hurt employment or wages among native workers in Miami at all. A huge spike in Russian immigration to Israel in the early 1990s appeared to give existing workers a nearly 9 percent raise.

Finally, the positive economic effects of immigration extend beyond just wages. Immigration increases property values, building wealth for many native-born workers (and, admittedly, raising rents for others). Increased immigration reduces the price of services provided by immigrants, such as gardening and housekeeping. There's some evidence that immigration even gets more women into the workforce by making it cheaper to hire people to watch after children and elderly relatives, and perform other homemaking tasks.

The claim that American-born workers would suffer from open borders is bogus

As economist Michael Clemens once told me, the effect of immigration on real wages for native workers is "definitely positive, without any doubt whatsoever." A recent evidence review by researcher David Roodman confirms this: While low-skilled immigration can make the existing low-skilled immigrant population worse off (though almost certainly not worse off than in their country of origin), Americans born here have very little to worry about, and a lot to gain.

It's true that all of our empirical research pertains to increases in immigration that are milder than pure open borders. The best we have to go on in guessing the effects of a total open-border policy are simulations. But those simulations show an increase in world GDP massive enough that it's fair to guess they'll hold harmless or help US workers — just as the data suggests smaller-scale immigration does. "This isn’t just trickle-down economics. It’s Niagara Falls economics," economist Bryan Caplan once told me. "If production in the world were to double, almost everyone is going to get enough of that doubling that they’re going to, in the end, be better off as a result."

If Bernie Sanders thinks we ought to give strict priority to the interests of immigrants already in the United States, even if doing so makes native-born workers and potential migrants worse off, then that's a very interesting opinion that I'd love to hear him attempt to defend. But the claim that American-born workers would suffer from open borders and increased immigration is bogus, and he should stop making it.

People are people, so why should it be that we treat potential immigrants so awfully?

The second problem isn't a matter of facts, but of values. As a US senator, Sanders believes he is obligated to put the interests of the United States — and of Vermont in particular — ahead of the interests of any other country. That means, for him, heavily discounting the interests of people in other countries.

Even if you think this makes sense, it doesn't make restricting immigration acceptable. Privileging the interests of Americans doesn't mean that US policymakers have the right to needlessly hurt foreigners. Not even the most ardent nationalist would say that the US has a right to, say, massacre 10,000 foreign civilians to save a single American life. And make no mistake: Using force to restrict access to the United States hurts foreigners dramatically.

Taking the idea that all people are created equal seriously entails supporting open borders

The philosopher Michael Huemer has a great thought experiment making this point. Imagine a man, Marvin, is starving to death, and goes to a marketplace to buy bread. Another man, Sam, forcibly stops him and prevents him from buying bread. Marvin starves to death.

That's wrong, right? And it's still wrong if the harm caused is less severe. Say Marvin isn't going to the marketplace to buy bread, but instead to sell it. If he sells it at that particular marketplace, he will make 15 times more money than if he sold it at the other marketplace in town. But Sam stops him, by force, from selling at the lucrative marketplace, forcing him to settle for the other market, where he makes 15 times less.

The analogy is not exactly subtle: Marvin is a potential immigrant (in this case from Nigeria; recall that moving from Nigeria to the US raises an average migrant's earnings 15-fold), and Sam is a US border patrol agent. If you think Sam is hurting Marvin by barring him from selling bread from the good market, you've got to think that border agents are hurting immigrants by keeping them from coming to work in the US.

I'm sure Sanders believes that Nigerian lives and Bangladeshi lives and Haitian lives matter. But if he does, then his views on immigration must change.

Maybe such harm would be justified if it prevents a major harm from befalling native-born Americans. But immigration does not harm native-born Americans on average. It helps them. It's hard to avoid the conclusion, then, that our border policy is causing major, unacceptable harm to immigrants. Even if you don't think the US is obligated to help immigrants, restricting immigration is wrong, because it actively hurts them.

Personally, I think the distinction between "not helping" and "hurting" isn't that meaningful. I do think the US is obligated to help immigrants. I think Bernie Sanders is obligated to weigh the interests of a poor potential Nigerian immigrant equally to those of a much richer native-born American. I think if he saw an immigrant drowning in a pond, he has just as much of a duty to rescue her as he would if she were a native-born American, and the same duty applies when he's voting in the US Senate. Taking that idea seriously — the idea that all people are created equal, and deserve to be treated as though their lives matter regardless of their place of birth — entails supporting open borders.

I don't doubt that Sanders thinks he takes equality seriously. I'm sure he thinks he's an egalitarian. I'm sure he believes that Nigerian lives and Bangladeshi lives and Haitian lives matter. But if he does, then his views on immigration must change.

Thanks to David Roodman for help summarizing the literature on immigration and native-born workers.