Personal experience. I've been with decent looking enough white women, but as a somewhat nerdy white guy, I've been able to pull a much higher caliber of Asian women (both Asian-American and native-born Asian). I'm sure there's a decent percentage of Asian women that just wouldn't date anyone outside their nationality or ethnicity, but there's also a decent percent that are more attracted to white men and that seem to select for the educated, nerdy types (like me!).