In the 1920s immigration to the United States was restricted with quotas that were designed to reduce the number of immigrants from Italy and Eastern Europe, then considered to be low-quality immigrants. One unintended consequence was that the number of immigrants scientists from this areas also declined. The awesome Petra Moser and Schmuel San have an excellent new paper documenting the cost on US innovation and patenting.
Canada which did not implement quotas did not see a similar decline. One interesting case study which is quite astounding in its way:
As you might suspect from the Erdos example, scientists in the US became less not more productive without the benefits of cooperation with Eastern European scientists.
Some of the scientists denied entry to the US in the 1920s went to Israel instead and innovated there so their genius was not entirely lost to the world.
Photo: Paul Erdos with Terrence Tao. Attribution, either Billy or Grace Tao, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons