There is an interesting paper on brain drain Why does the U.S. have the best research universities? Incentives, resources, and virtuous circles?

A demand for denominational sorting drove the creation of the nine American colonial colleges. For example, Massachusetts Puritans created Harvard to produce what they saw as a theologically sound education … Connecticut-based Puritans created Yale because they perceived that the Harvard of Massachusetts-based Puritans was too physically distant (in addition to too religiously liberal)

The authors attribute the following U.S. success to “reforms that began after the Civil War and enhanced the incentives and resources the system directs at research” which may be true.  Maybe the overall strategy of the paper is questionable, looking at biographies of Nobel prize winners  only- nevertheless the trend is clear that German impact is decreasing already in 1920 – while I always thought of an exodus of scientists only after 1933.

The U.S. does not spread so much money to  various non-university based “Großforschung” organisations and there is much more private sponsoring of U.S. universities, so monies are more concentrated.

The emergence of tenure seems to be important if you can make your living from your work and do not need to start campaigning like #IchbinHanna in Germany.