For example the United States constitution didn't happen quickly - there was a long period of people writing and discussing about such things, the age of enlightenment (I'm still grasping to understand how it happened, it seems so unlikely). There were also only under three million people living in USA when it became independent.
Ditto on Chinese immigrants in the later 1800s, Germans, etc. The point is that immigrants are always a minority in their host country, so they naturally have less of a base and thus power.
> And if their cultures clash with each other, you have a recipe for enduring intra-national strife.
Feels logical but also feels unproven.
So it is widely accepted that there is no/negligible genetic difference, but there are cultural differences at least when it comes to prosperity.
However it is also the case that cultures are extremely slow to change - multiple generations - almost to the actual speed of evolution. Essentially conceding the point that even if it isn't genetic there is some kind of heritable negative/missing positive traits which can last multiple generations.