The above photo shows migrants trying to climb into Melilla — an enclave still controlled by Spain but on territory Morocco claims. The reason why is obvious. Melilla has a GDP per capita of €16,426 (about $20,800 at today's exchange rates). That's low for Spain but nearly seven times higher than Morocco's ($3,109). And the residents (or, perhaps more likely, tourists) playing golf in the photo are probably doing considerably better still.
If they make it over, and are allowed to stay, the migrants will do not just a little better, but much, much better. Maybe not seven times better, but that number is not far off. It's pretty firmly established that workers in developed countries make much more than workers in developing countries doing identical labor. The Center for Global Development's Michael Clemens, Claudio Montenegro, and Lant Pritchett estimate that if the average Yemeni or Nigerian moved to the US and did the same work, they'd make about 15 times as much:
Melilla is poorer than the US, but Morocco is about as poor as Nigeria. The gains should be considerable. The gains are so large, in fact, that economic models trying to estimate the impact of completely open borders worldwide suggest the policy would increase world GDP by between 50 - 150 percent, much of which would redound to the world's poorest people, both through their own migration and through remittances sent back to their families.
So why don't we let people stay? It's kind of unclear. The case that immigration hurts rich countries like the US or Spain economically is so weak that even opponents tend not to make it. The case that it hurts low-skilled workers is heavily disputed and, in any case, addressable through transfer programs within rich countries. The economic case against immigration only makes sense if you're willing to put negative weight on the well-being of foreigners.
For such a poorly supported policy, we sure put a lot of effort into maintaining it. The US spends billions of dollars every year on armed guards and planes and drones to make sure the global poor stay poor. It wasn't always this way — US and European borders were more or less open until the outbreak of World War I — and it doesn't have to be this way in the future. We can, if we want to, "fast-forward to the world of the future where everyone can enjoy a First-World standard of living," as Bryan Caplan puts it. And that starts by tearing down the fence keeping migrants out of Melilla.
Thanks to Matt O'Brien for highlighting the photo.