It always make me smile when people pontificate that
Our people refuse too apply to human beings such elementary knowledge as every successful farmer is obliged to apply to his own stock breeding
and such like, then promptly advocate the absolute opposite of this 'elementary knowledge' which is that outbreeding is the way to obtain healthy stock. There is a reason why the term 'inbred' is rarely meant as a compliment!
The perennial bleating that the human species is going to the dogs through ill-breeding is, I suspect, fuelled by the firm belief of the middle class intellectuals who are doing the bleating that humanity would be better off if it consisted entirely of (insert colour appropriate to bleater), middle class intellectuals. This is almost certainly not so because if that was the case, evolution would have made it so. In reality nature generally takes care of this very well, because women are infinitely better at appropriate mate selection than US Presidents, German dictators and Eugenicists. Just as well, because if they weren't, humankind would have become extinct a very long time ago.
In that vein, have you ever noticed how some couples you wonder how they ever got together let alone stayed together long enough to have kids? Everyone told them don't do it, it's going to be a disaster - and it so often is - but the kids they produce are outstanding?