Also, the 'six times more productive' stat turns out to mean Math Olympiad medallists who migrate to the US are six times more likely to get a speaking slot at the International Congress of Mathematicians than people who stay at home; an impressive accomplishment but hardly what most people would call 'productivity'. On the slightly less arcane measure of weighted academic citations, migrants to the US have twice as many as migrants to other countries, or four times as many as those who stay at home.

Or put another way: moving to country with best funded and most prestigious research institutions turns out to be useful if you want a high-profile academic career