Open borders, although it’s a nice lodestar to shoot for, is a pretty radical policy and would seriously disrupt the current geopolitical status quo. People in first world countries think of immigration in terms of how it would affect those living around them, jobs, governmental programs, and politics. However, the biggest impact would likely be on the poor countries. No longer would petty dictators have their populations trapped, unable to escape oppressive rule. Without the Berlin Wall or the DMZ in Korea, I think it likely that the communist regimes would have collapsed on sheer mass exodus alone. And even significantly less oppressive regimes would still face lower populations without the first world cooperating with them in what are effectively international fugitive slave laws.
There are problems with moving quickly to such a different equilibrium. Economies move best slowly, adapting to small changes through trial and error. It might be useful to try to have some variance in the rate of immigration to different states to estimate the best rate of immigration and see which stresses develop without risking the whole country. To some degree we already do this by varying the level of enforcement of immigration laws, but it would be interesting to see some states have significantly higher immigration than is allowed currently just to see how they handle it. The problem with this is that the U.S. currently does not have a good system for varying immigration rates as anyone born within the U.S. is a citizen of the whole country, and we don’t regulate interstate movement. The irreversibility of immigration laws lowers support for immigration in the same way that making it difficult to fire workers raises unemployment. If you can’t go back, you will be far more risk averse. There are ways to reduce the risk to first world country citizens. Immigrants could not be allowed to vote, or take welfare money. Immigration could be limited to those with job offers. Immigration could be limited to relatively high skilled people. Immigration could be raised slowly at first and then more quickly later if it was successful. Immigrants could be ushered to areas suffering from underpopulation, such as Detroit, where the infrastructure could support far more than currently live there. There are many compromises immigration reformers could make to increase the palatability of higher immigration to its opponents, while keeping in mind the long run goal of open borders.
David Henderson on immigration