True, this will not avert marriage between cousins, but then there is nothing much wrong with marriage between cousins: The chance of a recessive deleterious gene emerging from such a match is small, and the advantages of genetic alliance to preserve complexes of genes that are adapted to work with one another probably outweigh it. (Quail prefer to mate with first cousins rather than with strangers.) Westermarck did not know that, of course, but it strengthens his argument, for it suggests that the only incestuous relations a human being should avoid are the ones between brother and sister,
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