IF YOU are a PhD student in America, there's a good chance that your undergraduate degree came from Tsinghua University in China. That's because Tsinghua and Peking Universities are now the top feeder schools for American PhD programmes. Chinese students have the largest presence in the natural sciences and engineering, and the better funded hard sciences have the most students. Thus, when you aggregate the number of PhD students, the Chinese universities prevail.

The increasing dominance of Chinese doctoral students does add to the popular perception that America will someday suffer a shortage of scientists and engineers. If that is indeed the case, why do so few Americans find themselves in PhD programmes?

I completed a PhD in a quantitative field where, as an American citizen, I was a minority among my fellow graduate students (though in my department, Europeans and South Americans dominated). There certainly existed a perception among students and faculty that American students did not have the math skills necessary for a graduate degree. There may be some truth to that. The curriculum I taught was much less quantitative than what I experienced in my foreign undergraduate studies.

American students who do have the skills necessary for a quantitative PhD might also be less likely to pursue graduate work, because these skills are in high demand. A clever graduate with strong quantitative skills can fetch a high salary right out of university. The alternative of seven years of indentured servitude to your adviser probably sounds less appealing to many recent graduates.

Students from China do not face such high-paying alternatives at home. Also, now that the number of H2 visas for skilled labour has decreased, PhD programmes provide a path to America for some. This helps explain why the number of foreign students in PhD programmes increased remarkably between 2001 and 2006. After completing their studies, most foreign-born students hope to stay in America.

And this is no bad thing. The resources at American universities are unparalleled elsewhere. This attracts the best students and faculty from all over the world, increasing the quality of education that all students receive. So long as America allows them to stay after they finish their studies, a greater proportion of foreign-born students may benefit everyone.