Why opposites attract

This is remarkable:

In 1995, a Swiss researcher showed why pheromones are so important in humans, too.  He had women sniff t-shirts worn by men, and asked which smelled best.  The results were startling: the women did not choose randomly, which was discovered by comparing the DNA of the women and men.  Instead, women overwhelmingly picked the scent of man whose histocompatibility complex (MHC)—the genes that forge our immune systems—differed from their own.  (Different MHC’s mean less immune overlap, and the increased likelihood of healthy, disease-resistant kids.)

I wonder if there is high variability among MHCs from people of the same race, or from similar nationalities.  If the variability is lower than inter-race comparisons of MHCs, this might help explain why some people prefer mates from different races, despite all the social pressures against it.