Matthew Yglesias in Slate has worked out some of what would happen if the United States opened up its borders:
According to Gallup there are 150 million people around the world who say they’d like to move permanently to the United States. Right now the United States has about 89 residents per square mile. Add another 150 million people and we’d be at around 135 people per square mile. How would that stack up in context? Well, France has 303 people per square mile and Germany has 593. Japan has 873. The Dutch have 1,287!
Of course, such a radical move would be anathema to most Americans (including most of those who themselves migrated to the country), but as Yglesias points out, ‘all those places have their share of problems (and so do we) but none of them are exactly post-apocalyptic hellscapes.’ Indeed, there may be large benefits to the US if it worked towards freeing up immigration:
The United States ran an open borders regime throughout the 19th century and we weren’t worse off for it. On the contrary, it laid the foundations for American greatness. Shifting back in that direction—with exceptions for dangerous criminals and other select problem types—over time seems perfectly feasible to me and would substantially increase overall human welfare.
We tend to value the welfare of our fellow countrymen more highly than that of those unlucky enough to be born in other countries (and particularly that of those born in poor countries), so open borders are likely to remain a taboo for now. For those interested in the sum of human wellbeing, though, it’s good to see such arguments getting an airing.