I'm going to dork out on you all as a biological anthropology nerd, but anthropolgists and the like have some answers to this question. I don't have citations to the studies here, and I'm going to write this from memory, so don't kill me if I give stats that are a little off.



First, the phenomenon definitely exists. If I remember right from my Asian-American studies classes in college, most people are most likely to date and marry someone within their own race. This has to do with both cultural and individual attraction, but can be influenced by a slew of factors, yada yada. When you look at Asians, the numbers vary a little bit by ethnicity (Chinese, Korean, Japanese, etc), but Asian-American women are something like 30% likely to marry a man outside their race, while Asian-American men are only .... 5%? 7%? ...I forget the exact number but it was astonishingly small compared to the number of women who married outside their race.



So why?



First, we can look at blanket studies about facial attractiveness. I'm not sure if the ones I remember studied AAs in particular, but their method was to show a whole lot of faces to people and have them rated on attractiveness, likeliness to date, whatever. For men, facial features that were marked by lots of testosterone were found to be most attractive as "guys to date." These features include heavy brow ridge, squared chin...picture syl stallone or something. However, after a certain point of testosterone-pumpedness, the guys became too "caveman" to be attractive. The "Guys for the long term/marriage" were a little more toward the feminine side of the spectrum as far as facial features. It should also be noted that women are most likely to cheat 1) at the time in their cycles when they're most likely to get pregnant, and 2) with men showing extra testosterone in their features.



Ok, so we've established that male faces with lots of testosterone are most attractive to most women.



Let's jump to recent human evolution, which is somewhat based on culture. It turns out that in Asian countries, sexual dimorphism is smaller than in European countries. That means the difference in body size between males and females is less in Asia than in Europe. I think the average across all humans is 18%, which means the average human female is 18% smaller than the average human male. That's a fairly large percentage among primate species (but think of Gorillas, whose males are 2x the size the females!)



Anyway, as far as making evolutionary sense, it's the most cost-effective to have a smaller body, right? You don't need to spend as many resources finding food to keep yourself going. Why would males need larger bodies (and more testosterone)? Because humans are not monogamous animals! Something in the cultural structure of most European countries led to more extra-marital affairs, less certainty of fatherhood for males, and better likelihood of high-testosterone males actually fathering children, whether