I awoke in the middle of the night last night suddenly understanding why the incest taboo is such a driving force. Of course, we all know the biological, genetic reason. Closely related people share a LOT of bad genes that would be masked in 99.9% of the cases of mating with a random stranger by the corresponding functional gene from the other partner. Thus, mating with your sister is a BAD idea - at least if you're planning on having kids with only one head, or a functioning brain.
However, the problem is that by nature, the closer and more knowledgeable about someone that you are, and the more kindred in spirit, the more perfect that person is a mirror of your soul, a natural mate and lover, as love is driven by the same urge to see "out there" in reality a clear, detailed reflection of ones self and ones actions in the world, as is every other significant human endeavor.
And who, then, is closer and more attuned to ones self than those of a similar age with whom one had lived ones entire life in close proximity? However, if this natural tendency is followed, disaster results.
The clever reader will now be able to put 2 and 2 together and see how this ties into the novelty seeking gene, and the unnatural neurotic drives for power over others that virtually every culture nourishes. The woman looks for a man who seems like a little boy, an idiot savant, brilliant, powerful and just a tad emotionally retarded and defenseless. The man is arroused, not by the perfect mirror, but by the challenge of the willful, playful, teasing Dominique Francon (Ayn Rand's "The Fountainhead").
These perverse, seemingly inexplicable impulses are the way that we preserve the incest taboo. Note that the cultures of the East that have the most family centered social arrangements also have the highest percentage (~85% of the population) carrying the novelty gene.