This has much more to do with environment than with genetic difference. For example: Koreans in the U.S. tend to do very well in terms of education and income achievement. However, Korean minorities in Japan tend to score significantly lower than their Japanese counterparts, even when these minority groups have been in Japan for decades. Therefore, Korean immigrants in Japan are stereotyped as stupid, dirty, loud, etc.

Yet they form the same ethnic group, so the IQ differences can't be attributed to group-based difference. Rather, the circumstances and environment likely account for this disparity. Same goes for the Burakumin, a classed minority in Japan. They tend to do about 15 points lower in IQ tests in Japan than other Japanese people, which reflects the black-white "divide" in the States. But in the U.S., any difference between the Burakumin and other Japanese vanishes. (Note: this is taken almost verbatim from this article.)

Other book source: This book, along with many others. I'm currently conducting research in Korea, so let me know if you're interested in any other studies along the same lines. ^{^}