While I agree with co-blogger Bryan that it would be desirable to let way more people into the United States, I haven't seen him answer in a satisfactory way some of the arguments against completely open borders.
Assume that the U.S. government decided to let anyone in who wanted to come, unless the person had a criminal record or carried a dangerous communicable disease. I would bet that these latter two disqualifiers would DQ under 10 percent of the relevant population. I could imagine that leading to an additional 300 million people coming into the United States within a couple of years. My impression is that Bryan could imagine it too.
That would effectively double the U.S. population. Now, under my scheme, the U.S. government would couple open borders with a 20-year residency requirement for U.S. citizenship and a requirement that one be a citizen in order to get any kind of welfare, including government schooling. That way, people wouldn't come here for welfare (I think most of them don't anyway) and we wouldn't worry about their voting away the very economic system that made this country attractive to them.
But Bryan and I both know that you can't always get what you want. The government is not some entity that he and I control. So what if the government did not couple open borders with this 20-year residency requirement or even a 10-year residency requirement? Would Bryan still advocate open borders? And would he worry at all that the new residents would vote away the goose that lays the golden eggs? And if he wouldn't worry about that, why wouldn't he worry? Inquiring minds would like to know.