In a survey of scientists from 16 countries (http://spectrum.ieee.org/at-work/tech-careers/the-global-bra... ), the US is the top destination from 13 of the 15 others and the #2 choice from the other two. If you are a Canadian scientist, there is a 16% chance (https://www.reddit.com/r/dataisbeautiful/comments/37lgxg/the... ) that you will move to the US. That's not "16% of all Canadian scientists that move out of the country move to the US". Let me repeat: *16% of all Canadian scientists move to the US.* They're also likely to be among the top Canadian scientists, too.

By comparison, 5% of all American scientists move to another country, of which 32% go to Canada, so about 1.6-1.7% total. Since the US has nine times more people, that means that in absolute numbers the 1.7% of American scientists is about equal to the 16% of Canadian scientists, but there is no reason to think that the 1.7% makes up the top tier of American scientists; why would the best move north of the border? In other words, the US is receiving the best of Canadian scientists in exchange for an equal number of its non-best.