[Quinn]

My limited knowledge of animal husbandry, genetics, and some sociology thrown in (a chance for me to show off):

Mating one individual to it's own sibling is common in animal husbandry (line breeding). It helps to stabilize certain traits (recessive). The interesting thing is, dog, cat, bird and undoubtedly fish breeders all do this... why hasn't it happened in humans?

It has. Before western culture (judeo-christian norms) began to take over the world, there were essentially five kinship systems. Ours is known as the eskimo system: grandma, grandpa, aunt, uncle, mom, dad, brother, sister, cousins, kids, nieces and nephews, etc. One of the others was known as hawaiian. Mom, dad, brother, sister, son, daughter. No one else. So anyone in your generation is your brother or sister. Anyone above you is mom or dad. Anyone below you is son or daughter. So yes, you do end up procreating with a sibling (and they were polygamous). Interestingly, this worked very well for several thousand years until the missionaries arrived. Interestingly, rather than having kids with one blue eye and one green eye, or two mouths, as the stories go, you simply had an equal chance of either transmitting "good" genes or "bad" genes. One of my professors really got off on scaring us kids with that story. But it's true.

So what I'm getting at here is that basically line breeding isn't as bad as people think, whether you're talking animals or humans. The reason why we don't do it? Because we think it's gross. The reason we think it's gross? In my opinion, like so many other religious traditions, it's been proscripted (proscripted meaning something you should not do, contrary to what you might think). See, you end up with two kids. One is really smart, strong, and can take down a mammoth with his or her bare hands. The other has defects. So do you cull the defective one? For a while maybe. Then something called rationalization came along, and it got thrown into primitive religion, and ancient judaism was born (and from that, christianity). So that's why we don't make babies with our siblings (and I'm fine with that).

Appendix A
Just to give examples of where line breeding can be good and bad:
Good - Lots of colours of dogs, cats, gerbils...
Bad - Bananas are actually fairly close to extinction (not enough variety between species)

Anyone with a more extensive background on this topic than me should feel free to correct me.