Any visa policies that restrict entry by highly productive foreign students are a significant barrier to science and ultimately to innovation and growth. That is one of the conclusions of research by Professors Eric Stuen, Mushfiq Mobarak and Keith Maskus, published in the latest issue of the Economic Journal. Their study of 700,000 postgraduates in the science and engineering laboratories of the top US universities finds that American students and foreign students are both highly significant contributors to the development of scientific knowledge. But greater diversity in the origins of foreign students raises their joint contribution to knowledge. These findings imply that visa restrictions limiting the entry of high- ability foreign students – as well as visa policies that prioritise students’ ability to pay tuition fees over their technical merits – would significantly undermine scientific output.