The USA produced 26% of the most cited research papers in the world in 2012.
We have less than 5% of the population.
And, some not so good news...
Many major European countries get considerably more “bang for the research buck.”
Individually, they produce far fewer papers.
However, their rate of most cited papers is often higher than the USA rate, for instance, almost twice as high in Switzerland.
This indicates to me that America may be wasting a considerable amount of research money compared to other highly developed nations.
Or, it may indicate there is a law of diminishing returns in R&D.
Or, it may indicate that smaller countries focus particularly well on a limited number of projects.
My guess - there is only so much world class intellectual talent.
As more and more of that talent stays in it’s home country, America may be funding more and more second rate talent.
Global warming, ocean acidification, human embryonic stem cell research, tons of social science nonsense, etc.