Edited

Japan is a little smaller than the state of california.

The population of california is about 36,457,549.

You estimated the population of japan at 127.000.000.

I do not believe a crowded population is a good assessment for IQ's.

There are many studies that have started controversy.

Example:

Rushton and Jensen examined 10 categories of research evidence from around the world to contrast "a hereditarian model" (50% genetic-50% cultural) and a culture-only model (0% genetic-100% cultural). Their article "Thirty Years of Research on Race Differences in Cognitive Ability" was published in the APA journal Psychology, Public Policy and Law showing evidence that they believe supports the hereditarian model. Rushton and Jensen believe that the best explanation for the gap is that 50%-80% of the group differences in average US IQ is genetic.

Other evidence, such as the Minnesota Transracial Adoption Study, certain racial admixture studies, behavior genetic modeling of group differences, "life-history" traits, and evolutionary explanations have also been proposed to indicate a genetic contribution to the IQ gaps and explain how these arose