MacAskil discusses this in a section titled “international labor mobility” but does not mention “open borders” or draw the distinction you have. He writes:
“Increased levels of migration from poor to rich countries would provide substantial benefits for the poorest people in the world, as well as substantial increases in global economic output. However, almost all developed countries pose heavy restrictions on who can enter the country to work.
…
Tractability: Not very tractable. Increased levels of immigration are incredibly unpopular in developed countries, with the majority of people in Germany, Italy, the Netherlands, Norway, Sweden, and the United Kingdom favoring reduced immigration.”
As the quote you provided shows, labor mobility was rated as “not very tractable”, not “intractable”. Moreover, labor mobility was given that rating because it was judged to be politically infeasible, in light of the low popularity of even modest migration reform proposals, and not because we lack evidence from RCTs, or due to blindness to the mechanics of political change. So I think you are mischaracterizing what was said in the book.
MacAskil discusses this in a section titled “international labor mobility” but does not mention “open borders” or draw the distinction you have. He writes:
“Increased levels of migration from poor to rich countries would provide substantial benefits for the poorest people in the world, as well as substantial increases in global economic output. However, almost all developed countries pose heavy restrictions on who can enter the country to work. … Tractability: Not very tractable. Increased levels of immigration are incredibly unpopular in developed countries, with the majority of people in Germany, Italy, the Netherlands, Norway, Sweden, and the United Kingdom favoring reduced immigration.”
As the quote you provided shows, labor mobility was rated as “not very tractable”, not “intractable”. Moreover, labor mobility was given that rating because it was judged to be politically infeasible, in light of the low popularity of even modest migration reform proposals, and not because we lack evidence from RCTs, or due to blindness to the mechanics of political change. So I think you are mischaracterizing what was said in the book.