[haikuginger]

When I read this one obvious explanation would be that IQ tests are designed with biased assumptions based on the cultural environments in which they were designed, and that those biased assumptions become more and more foreign as more generations pass since the test was originally designed.

See? I can speculate baselessly too.

[Symmetry]

There's certainly an advantage to taking IQ tests if you come from a culture that values abstract thoughts relatively highly compared to concrete thoughts. This is all based on Norwegian data and if there were some huge cultural shift in Norway I might not know. But usually when you see big changes it's adding iodine to the salt, or lead in the environment, or famines, or disease or something like that. Seriously, iodizing salt added 10 points to the IQs of Americans around the great lakes where iodine deficiency had been a big problem.

And generally if it was a matter of shifting cultural environments I'd hope that would be obvious from looking at the subtest scores.