Actually, inbreeding once has never been proven to cause birth defects. Let me explain where that myth came from. Lets say you take a brother and sister, they both have horrible genes, there ugly, fat, stupid, and prone to disease. If they bred, there offspring would carry on ONLY there genes, and the bad traits would be more prominent, basically doubled. Lets say there offspring had incestuous sex with eachother, and birthed some babies. The babies would carry those same traits, and it would be much more prominent and stronger. If this was to keep going, you would have a bunch of retarded diseased fuck ups. NOW, lets say you took a brother and sister, with extremely good traits. Literally near perfect genes. If they bred, there offspring would carry on ONLY there genes, and the good traits would be more prominent, basicaly doubled. If there offspring bred incestuously with eachother, and so on and so on. All while not letting any fuck ups continue to breed. You would end up with extremely healthy, very intelligent, disease resistant, long living, humans. Heres the problem, finding people with extremely good genes is very rare, and getting them to breed incestuously, and getting there kin to breed with eachother, and repeating that step, is even harder. But still, inbreeding humans with good genes is a way to create super humans, but science won't even touch it because its so damn forbidden.I mean, we do it with dogs and other mammals, it does work. Like if 95% of people decided to start inbreeding, in the way I described, they would just end up with disease prone idiots. Finding people with near perfect genes to do this, with very, very little flaws, is very rare.