According to this analysis based on PISA scores immigrants are decreasing the national IQs of the West. To put it crassly they're making us dumb.
There's probably a flaw in the methodology but living in Toronto for as long as I have I'm inclined to think it's correct for the most part. If our immigration system is designed to filter for the "best and brightest" it's apparent it's not doing it's job. Or the more reasonable explanation is that Canada doesn't attract the world's "best and brightest" at all and has to settle for what washes up on our shores which tend to be the third-world's C and D students. When you're the safety school country to America's Harvard or the U.K.'s Oxford you're forced to settle for what comes your way.
And if the mostly third-world immigrants Canada attracts are indeed the developing world's "best and brightest" then it explains why the the third-world is in such a sorry state.
Come to think of it allowing immigrants from the third-world to settle in the West is a lose/lose situation. For one, these mostly unremarkable people add no value to the Western countries they settle in since the West already has high standards of achievement to which their contributions would be negligible at best. And secondly, by being the "best and brightest" of their country and removing themselves from it makes their native countries dumber and worse off. Everyone loses in this scenario except the immigrant.
It's clear to me Canada and the developing world are better off if Canada doesn't accept immigrants from the third-world at all.