An empirically testable hypothesis. How many applications for naturalization are there per country, compared to where that country ranks in the indices of freedom, prosperity and others. You'd probably also want to weight it for factors such as size and extant population.
At a glance, the US gets over 800,000 applications per year. China by contrast gets about 1500 application per year.
More freedom, fewer taxes and economic constraints seems to be something people like. I hope we don't forget it.