<p>It has been pointed out to me that there are citizenship restrictions on many graduate fellowships, so the “preferences” might actually be restrictions instead. When I started as a faculty member, I was informed by my program officer at the National Science Foundation that I could support graduate students of any nationality but Japanese on my research grants–so you can guess when that was. Now there are no formal restrictions of that sort, but I wouldn’t be surprised if there is some subtle encouragement to support Americans with research assistantships from a professor’s grants. In fact, when one files the progress reports with NSF, there are questions about the citizenship of all supported on the grant.</p>