Phew, a lot of interesting stuff. First of all, studies have shown that we are attracted, at a subconscious level, to people with different
Genotypes than us. How is this possible you ask? We smell others, particularily the opposite sex, and Pheromones carry genetic information about ourselves. ALso, its a cultural universal that brothers/sisters are taboo, and for this reason it doesnt matter whether it is legal or not, members in society in general won't practice sibling incest. The article on wikipedia explains a lot, albeit offering two sides of the equation. I'm in a biological Anthropology class, and I remember hearing that incest, not neccessarily between brother and sister, would work against the homozygous recessive type, all of the offspring with homozyous recessive traits would die, thus leading in a stronger presence of the dominant allele. On the other hand, so much of the early population would die that some suggest that the long term effects wouldnt be realized. Reading further suggests that evolutionary psychologists theorize that we have built in aversions to our siblings/parents.
For those of you against incest, where do you draw the line? Are cousins fair game, because in most cultures this happens frequently.
My personal belief is that it is unnatural for siblings to mate, this is from a religious perspective but there is evidence that we, as well as other species, are naturally unattractive to our siblings.
Therefore, it is not surprising that inbreeding is uncommon in nature, and most sexually reproducing species have mechanisms built in by natural selection to avoid mating with close kin. Pusey & Worf (1996) and Penn & Potts (1999) both have found evidence that some species possess evolved psychological aversions to inbreeding, via kin-recognition heuristics.