I've looked at the question of PISA scores before (here). But a recent Economist article about Neanderthals has given me a new idea: they test for Neanderthal genetic inheritance.
Neanderthals and humans interbred, each bringing slightly different things to the party. Neanderthals had certain advantages, in particular because they lived in colder places (Finland, anyone? Scandinavia in general?). Curiously (as Neanderthals are not known to have lived in East Asia), "on average, East Asians have more Neanderthal DNA than Europeans do". So that's China, Singapore etc all accounted for too.
Well, it's a theory, anyway.
The article is here. It is illustrated with some unkempt people who presumably were all very good at factorising equations but not so good at the skills you needed in the Stone Age.