That popular belief is also a myth.
No doctor wanting to keep his doctorate would confirm it as a true thing.
You will only find such info in sensationalist magazines, articles and random people in blogs. There is no medical proof of incest being that damaging to the human race or else wed have been long turned to mutant idiots long ago.
It was a very common practice not even 200 years ago to marry within the family.
It is religion that took this popular belief and turned it to sin(mutants/ sick). Which is also a myth. Religions of old relied on incest to keep high level officials in line and always in power. These days they condemn incest and use political power to be the puppet master they always were.
Aside from religious delusions, there's the medical delusions I mentioned.
Look to nature as your guide to reality. Animals often practice incest. It has been proven that the longest living as a species also practiced incest.
The draw to look outside ones family/pride whatever is to create variety to the gene pool, yet it is perfectly acceptable to stay inside for a few generations. Then peek out and spread the genes outward again. Humanity doesn't NEED THAT much a variety to keep it going and healthy.
