I’ve contended for some time that the Japanese IQ advantage over those of European stock may well be due to nutrition, notably the provision in the diet of more omega-3 fatty acids. See, for example, Docosahexaenoic acid and human brain development: Evidence that a dietary supply is needed for optimal development:
Humans evolved a uniquely large brain among terrestrial mammals. Brain and nervous tissue is rich in the omega-3 polyunsaturated fatty acid (PUFA) docosahexaenoic acid (DHA). Docosahexaenoic acid is required for lower and high order functions in humans because of understood and emerging molecular mechanisms…. Most current evidence suggests that the DHA-rich human brain required an ample and sustained source of dietary DHA to reach its full potential.
It’s well known that most Westerners eat a diet abysmally low on DHA and other omega-3 fats. In Japan the level is much higher due to consumption of fish and other seafood.
How high is the Japanese IQ? In 1982, Richard Lynn asserted that the younger generation of Japanese had an IQ of 111. IQ in Japan and the United States shows a growing disparity.
Next question: is the IQ difference between Japanese and whites mainly genetic? To see, take a look at Japanese immigrants to the U.S. Here is James Flynn (of the Flynn effect) stating that Japanese in Hawaii in the 1960s had verbal IQs of about 94 to 97, but with a somewhat higher score on “math achievement” than Caucasians. Link.
So if all this is correct – and of course it will be subject to possibly major revision, and I’m no IQ expert – then the Japanese in Japan may well have higher IQs due to superior brain development in infancy, caused by higher consumption of DHA, and other omega-3 fats.