That is undoubtly one of the drivers but to my point, it would only matter if it was a driver that might explain why non-residents are more likely to get a PhD

That is undoubtly one of the drivers but to my point, it would only matter if it was a driver that might explain why non-residents are more likely to get a PhD than residents. I could see that being true. Perhaps non-residents value a PhD for family or cultural reasons more than residents do.