Biological instinct in female to seek out a mate outside of the group.

I can offer some research that may add some support to the idea. To be valid though we'd have to assume that there's more likelihood of dissimilar MHCs the farther away from your home group you are. Which seems reasonable.

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Opposites attract -- how genetics influences humans to choose their mates
Vienna, Austria: New light has been thrown on how humans choose their partners, a scientist will tell the annual conference of the European Society of Human Genetics today (Monday May 25). Professor Maria da Graa Bicalho, head of the Immunogenetics and Histocompatibility Laboratory at the University of Parana, Brazil, says that her research had shown that people with diverse major histocompatibility complexes (MHCs) were more likely to choose each other as mates than those whose MHCs were similar, and that this was likely to be an evolutionary strategy to ensure healthy reproduction.

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Females' preference for MHC dissimilar mates has been shown in many vertebrate species, including humans, and it is also known that MHC influences mating selection by preferences for particular body odours. The Brazilian team has been working in this field since 1998, and decided to investigate mate selection in the Brazilian population, while trying to uncover the biological significance of MHC diversity.

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The scientists studied MHC data from 90 married couples, and compared them with 152 randomly-generated control couples. They counted the number of MHC dissimilarities among those who were real couples, and compared them with those in the randomly-generated 'virtual couples'. "If MHC genes did not influence mate selection", says Professor Bicalho, "we would have expected to see similar results from both sets of couples. But we found that the real partners had significantly more MHC dissimilarities than we could have expected to find simply by chance."

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Dienekes’ Anthropology Blog: MHC-dissimilar mating in Brazil

Thanks :-)DNA analysis is unearthing some inconvenient truths about our mating habits.

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A man can be absolutely sure if a child is his or not - and it's not a trivial matter. Lest anyone think that there is nothing but misogynistic bigotry behind the concern over accurate claims of paternity, a 1999 study by the American Association of Blood Banks discovered that in 30 percent of 280,000 blood tests performed to determine paternity, the man tested was not actually the biological father of his children.

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If we accept the hypothesis that women are programmed to seek diversity, why would they stick to one partner for all their children? Much better to get themselves a steady partner that can provide for the family, have a child by him, then seek another partner with different genes just in case?Cynical moi?