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We've known for quite a while that people, women in particular, tend to be attracted to men with different HLA profiles than theirs. This is particularly true when they are close to ovulation. When they are pregnant their preference changes to those with very similar profiles. The mechanism for this selection is smell.
This effect is particularly pronounced in groups like the Amish and Chassidim who practice severe inbreeding. Any shred of heterozygocity is a matter of group survival. It is also a reason why from a purely biological point of view the best thing that could happen to these groups would be successive invasions by the Mongol hordes, Vikings and Dayak pirates.
I hesitate to ascribe "reasons" to evolutionary phenomena, but a significant effect is that children sired by such men will have more diverse defenses against disease. It may also be a proxy for increased heterozygocity in general which confers many advantages. The second part, prefering the smell of close kin during pregnancy, may well be an artifact of our evolutionary past when we practiced female philopatrie - women sticking close to the family groups they were born into as many mammals do. Staying with close kin helped insure survival during pregnancy.
It must also be noted that female gorillas and chimps will travel to other troops at great personal risk in order to mate with males who are not their close relatives.
