Genetics
I am no expert in genetics or biology, but in every species I have observed, especially dogs, broad genetic mixing has resulted in hardier and smarter offspring. I don't expect humans are any different.
As irrefutable scientific evidence, I offer the fact that many of the hottest looking women in the world have a mixed ethnic heritage.
As for Amish websites, there are plenty. Here's a good one, including a link on Amish genetics. ;)