This is the title of a fairly interesting paper, the conclusions of which I would summarise as follows: after the collapse of the Soviet Union there was a sudden drop in the number of papers published by Americans working in fields which had been popular in the Soviet Union, which did not happen in fields which were unpopular there. Their papers were also less cited, and they moved to less prestigious institutions. Collaborating with Soviet immigrants afforded some but not total protection from this effect. Remarkably (and not entirely irrelevantly for me; I’m actually a data point in the paper, and my advisor is actually mentioned by name) this effect was transmitted to students: students after the fall of the Soviet Union wrote more (and more cited) papers if their advisor had immigrated to the US than if they had an American advisor who works topics popular among Soviets (with students that had American advisors working in topics unpopular with Soviets were in between).
This is obviously a very crude analysis (for example, “fields” means top level AMS Subject Class groups, which we all know are deeply flawed), but it’s still a very interesting, and to me at least slightly counter-intuitive conclusion. The authors suggest that the immigration of Soviet mathematicians to the US had a slightly negative effect on the number of papers written by all mathematicians in the US (the immigrants didn’t write enough to make up for the drop amongst American authors), which is not at all what I would have expected.