I sincerely doubt that it's genetic. There just simply wasn't enough population isolation to generate that level of genetic variation over the timescale involve

I sincerely doubt that it's genetic. There just simply wasn't enough population isolation to generate that level of genetic variation over the timescale involved (even a couple kiloyears isn't that long in evolutionary terms.) In the early twentieth century, the world also marveled at Jewish athletes. That isn't exactly the stereotype anymore. If you perform even a casual examination of Ashkenazi Jewish culture in the United States (if such a monolithic group could be said to exist), you'd see a few fairly common elements: bilingual study from an early age, a strong focus on education as the path to wealth, strong family ties creating a social safety net, kin-group contributions both tangible and intangible, and among the wealthy, all the factors that go along with having wealthy parents. Wealth confers a bevy of physical advantage: early childhood nutrition, sanitary living conditions, and a stable family environment all come to mind. Finally wealth conveys a package of invisible advantages: better parenting strategies, more parental involvement, intellectual confidence, social belonging, really an entirely different series of mental models for understanding and navigating the world. People underestimate exactly how much parents' shape their children's destiny. Data from admissions programs, studied during ththe affirmative action and legacy admissions arguments, demonstrates the single most powerful predictor of a student's likelihood of finishing undergraduate education isn't GPA, SAT, or high school institution; it's the level of education completed by their parents. The point is that it is very difficult to disentangle the effects of any combination of wealth, genetics, epigenetics, and a particular culture, especially compounded over generations.