Via Skeptical Analysis:
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Natural selection tends to avoid incest. Incest — more properly, inbreeding — allows recessive genetic traits to accumulate, often to the detriment of affected individuals. If a child gets a bad gene (doesn’t make a needed protein) from one parent, it’s best if the other parent doesn’t also contribute the bad gene.
Popular literature suggests wild populations, such as wolves, seek mates from outside their own packs. Also, primitive peoples may raid neighboring clans for wives, and friendly exchanges of eligible women between ruling European families provided genetic diversity while maintaining royal status.
Cultural and intellectual incest is a problem of a slightly different nature. Lack of cultural diversity can deprive a nation of the benefits of innovation and can also result in the development and retention of perverse cultural traits. Open societies are the fix. Honor killings within some European societies have lost fashion as a result of the cultural dilution that resulted from advances in communications and exchange of populations in the twentieth century.