I actually read an article about this a while ago. Being asian, i'm no longer offended when people say things like this.
It's not actually the race itself, but rather the culture. The omega oils found in fish stimulates brain development in utero and in the early stages of infancy. Basically, if you eat a decent amount of fish your child will be smarter. The bad side of this is that if you eat too much, you'll get mercury poisoning and your child will end up being retarded.
The Asian diet often has some sort of seafood in it, whether it be fish, shrimp or anything else. This leads to improved brain development and overall gain in intelligence.