More than 95% of people in the world are not American, and good American universities have access to the top talent. If the US is doing a competent job of hiring the best people then the majority of these researchers will not be American-born.
1. American colleges are still overwhelmingly the best in the world. https://en.wikipedia.org/wiki/Academic_Ranking_of_World_Univ...
2. 96% of students at American colleges are American. http://www.usnews.com/education/best-colleges/articles/2014/...
More than 95% of people in the world are not American, and good American universities have access to the top talent. If the US is doing a competent job of hiring the best people then the majority of these researchers will not be American-born.