Some disorders (usually the pretty bad/rare ones) are only expressed through the parents having a recessive gene and both of them passing it on to the offspring. The chances of that between two non-related people is much more slim, than between two related people (2 related people have similar genetics and might both be carriers of the recessive gene). Of course, while a sister might have the recessive gene, the brother may not, and its just as rare as two unrelated people. However, I'm pretty sure that's where the idea of deformed/retarded/etc. children through incest comes up. Through phermones, physical appearance, etc. we instinctively seek out a mate that is different than us, especially when it comes to immune system. Plus the whole instinct of expanding the gene pool. Of course, our gene pool as a species is pretty broad right now the way it is, so social factors play more of a part in one's aversion to incest -- screwing up family dynamics and such.
Anyway, to be vaguely on topic, I'm in the boat that says two people didn't just spring up and make babies, incest, etc. to populate the earth. Although that goes hand in hand with evolution and there's no way to really argue it from a Creationists standpoint. And Evolution will never be proven -- science doesn't prove, it just eliminates other possibilities. Creationism/God/etc. are impossible to disprove (or prove for that matter, as there is nothing tangible/measurable about them), which means its "possibility" cannot be disproven scientifically. Of course, nothing in science is absolutely 100%, but I think sometimes its necessary to make assumptions as a scientists or we'd all be sitting here believing in absolutely nothing and being completely unsure about everything around us.
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